syzbot

panic: Decrementing non-positive ref count 0xc0004bd930, owned by vfs.Filesystem

goroutine 292 [running]:
panic({0xfe2ce0?, 0xc0005f09e0?})
	GOROOT/src/runtime/panic.go:1017 +0x3ac fp=0xc00095b398 sp=0xc00095b2e8 pc=0x43a72c
gvisor.dev/gvisor/pkg/sentry/vfs.(*FilesystemRefs).DecRef(0xc0004bd930, 0xc0005ff820?)
	bazel-out/k8-fastbuild/bin/pkg/sentry/vfs/filesystem_refs.go:126 +0x10c fp=0xc00095b400 sp=0xc00095b398 pc=0x75580c
gvisor.dev/gvisor/pkg/sentry/vfs.(*Filesystem).DecRef(0xc00095b480?, {0x1442c78?, 0xc0006c4a80?})
	pkg/sentry/vfs/filesystem.go:77 +0x4b fp=0xc00095b440 sp=0xc00095b400 pc=0x75504b
gvisor.dev/gvisor/pkg/sentry/vfs.(*VirtualFilesystem).unlockMounts(0xc00053cc58, {0x1442c78, 0xc0006c4a80})
	pkg/sentry/vfs/vfs.go:1030 +0x16e fp=0xc00095b4f0 sp=0xc00095b440 pc=0x7702ae
gvisor.dev/gvisor/pkg/sentry/vfs.(*Mount).destroy.func1()
	pkg/sentry/vfs/mount.go:826 +0x28 fp=0xc00095b518 sp=0xc00095b4f0 pc=0x75d4e8
gvisor.dev/gvisor/pkg/sentry/vfs.(*Mount).destroy(0xc00050c630, {0x1442c78, 0xc0006c4a80})
	pkg/sentry/vfs/mount.go:839 +0x1e2 fp=0xc00095b5b8 sp=0xc00095b518 pc=0x75d442
gvisor.dev/gvisor/pkg/sentry/vfs.(*Mount).DecRef(0xc00050c630, {0x1442c78, 0xc0006c4a80})
	pkg/sentry/vfs/mount.go:820 +0x65 fp=0xc00095b5e0 sp=0xc00095b5b8 pc=0x75d225
gvisor.dev/gvisor/pkg/sentry/vfs.(*VirtualFilesystem).unlockMounts(0xc00053cc58, {0x1442c78, 0xc0006c4a80})
	pkg/sentry/vfs/vfs.go:1030 +0x16e fp=0xc00095b690 sp=0xc00095b5e0 pc=0x7702ae
gvisor.dev/gvisor/pkg/sentry/vfs.(*VirtualFilesystem).UmountAt.func2()
	pkg/sentry/vfs/mount.go:578 +0x28 fp=0xc00095b6b8 sp=0xc00095b690 pc=0x75c7e8
gvisor.dev/gvisor/pkg/sentry/vfs.(*VirtualFilesystem).UmountAt(0xc00053cc58, {0x1442c78, 0xc0006c4a80}, 0xc000521800?, 0xc00034fa00?, 0xc00095b80c)
	pkg/sentry/vfs/mount.go:602 +0x2fd fp=0xc00095b780 sp=0xc00095b6b8 pc=0x75c5fd
gvisor.dev/gvisor/pkg/sentry/syscalls/linux.Umount2(0xc0006c4a80, 0x0?, {{0x7f7040320a00}, {0xa}, {0x0}, {0xffffffff}, {0x0}, {0x0}})
	pkg/sentry/syscalls/linux/sys_mount.go:164 +0x2ab fp=0xc00095b918 sp=0xc00095b780 pc=0xad346b
gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).executeSyscall(0xc0006c4a80, 0xa6, {{0x7f7040320a00}, {0xa}, {0x0}, {0xffffffff}, {0x0}, {0x0}})
	pkg/sentry/kernel/task_syscall.go:142 +0x673 fp=0xc00095bc58 sp=0xc00095b918 pc=0x992c13
gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).doSyscallInvoke(0xc0006c4a80, 0xc0006925a8?, {{0x7f7040320a00}, {0xa}, {0x0}, {0xffffffff}, {0x0}, {0x0}})
	pkg/sentry/kernel/task_syscall.go:322 +0x45 fp=0xc00095bcb8 sp=0xc00095bc58 pc=0x993d85
gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).doSyscallEnter(0xc00095bcf8?, 0x126afd0?, {{0x7f7040320a00}, {0xa}, {0x0}, {0xffffffff}, {0x0}, {0x0}})
	pkg/sentry/kernel/task_syscall.go:282 +0x59 fp=0xc00095bd18 sp=0xc00095bcb8 pc=0x993a99
gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).doSyscall(0xc0005d8410?)
	pkg/sentry/kernel/task_syscall.go:257 +0x2d5 fp=0xc00095be10 sp=0xc00095bd18 pc=0x993835
gvisor.dev/gvisor/pkg/sentry/kernel.(*runApp).execute(0xc00062e380?, 0xc0006c4a80)
	pkg/sentry/kernel/task_run.go:269 +0xfb7 fp=0xc00095bf48 sp=0xc00095be10 pc=0x987b17
gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).run(0xc0006c4a80, 0x8)
	pkg/sentry/kernel/task_run.go:98 +0x1ef fp=0xc00095bfc0 sp=0xc00095bf48 pc=0x9864af
gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).Start.func1()
	pkg/sentry/kernel/task_start.go:391 +0x25 fp=0xc00095bfe0 sp=0xc00095bfc0 pc=0x991525
runtime.goexit()
	src/runtime/asm_amd64.s:1650 +0x1 fp=0xc00095bfe8 sp=0xc00095bfe0 pc=0x471241
created by gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).Start in goroutine 290
	pkg/sentry/kernel/task_start.go:391 +0xe5