PF_BRIDGE: br_mdb_parse() with unknown ifindex

=====================================
WARNING: bad unlock balance detected!
4.15.0+ #288 Not tainted
-------------------------------------
syz-executor2/28510 is trying to release lock (mrt_lock) at:
[<00000000c0ed6633>] ipmr_mfc_seq_stop+0xe1/0x130 net/ipv6/ip6mr.c:553
but there are no more locks to release!

other info that might help us debug this:
1 lock held by syz-executor2/28510:
 #0:  (&p->lock){+.+.}, at: [<0000000056de2f52>] seq_read+0xd5/0x13d0 fs/seq_file.c:165

stack backtrace:
CPU: 1 PID: 28510 Comm: syz-executor2 Not tainted 4.15.0+ #288
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 print_unlock_imbalance_bug+0x12f/0x140 kernel/locking/lockdep.c:3484
 __lock_release kernel/locking/lockdep.c:3691 [inline]
 lock_release+0x6fe/0xa40 kernel/locking/lockdep.c:3939
 __raw_read_unlock include/linux/rwlock_api_smp.h:225 [inline]
 _raw_read_unlock+0x1a/0x30 kernel/locking/spinlock.c:248
 ipmr_mfc_seq_stop+0xe1/0x130 net/ipv6/ip6mr.c:553
 seq_read+0xc42/0x13d0 fs/seq_file.c:277
 proc_reg_read+0xef/0x170 fs/proc/inode.c:217
 __vfs_read+0xef/0xa00 fs/read_write.c:411
 vfs_read+0x11e/0x350 fs/read_write.c:447
 SYSC_pread64 fs/read_write.c:611 [inline]
 SyS_pread64+0x15b/0x190 fs/read_write.c:598
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x453299
RSP: 002b:00007f9bfe4a6c58 EFLAGS: 00000212 ORIG_RAX: 0000000000000011
RAX: ffffffffffffffda RBX: 000000000071c010 RCX: 0000000000453299
RDX: 000000000000000e RSI: 00000000207afff2 RDI: 0000000000000014
RBP: 00000000000002a5 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000212 R12: 00000000006f3018
R13: 00000000ffffffff R14: 00007f9bfe4a76d4 R15: 0000000000000006
BUG: sleeping function called from invalid context at lib/usercopy.c:25
in_atomic(): 1, irqs_disabled(): 0, pid: 28510, name: syz-executor2
INFO: lockdep is turned off.
CPU: 1 PID: 28510 Comm: syz-executor2 Not tainted 4.15.0+ #288
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 ___might_sleep+0x2b2/0x470 kernel/sched/core.c:6114
 __might_sleep+0x95/0x190 kernel/sched/core.c:6067
 __might_fault+0xab/0x1d0 mm/memory.c:4532
 _copy_to_user+0x2c/0xc0 lib/usercopy.c:25
 copy_to_user include/linux/uaccess.h:155 [inline]
 seq_read+0xcb4/0x13d0 fs/seq_file.c:279
 proc_reg_read+0xef/0x170 fs/proc/inode.c:217
 __vfs_read+0xef/0xa00 fs/read_write.c:411
 vfs_read+0x11e/0x350 fs/read_write.c:447
 SYSC_pread64 fs/read_write.c:611 [inline]
 SyS_pread64+0x15b/0x190 fs/read_write.c:598
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x453299
RSP: 002b:00007f9bfe4a6c58 EFLAGS: 00000212 ORIG_RAX: 0000000000000011
RAX: ffffffffffffffda RBX: 000000000071c010 RCX: 0000000000453299
RDX: 000000000000000e RSI: 00000000207afff2 RDI: 0000000000000014
RBP: 00000000000002a5 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000212 R12: 00000000006f3018
R13: 00000000ffffffff R14: 00007f9bfe4a76d4 R15: 0000000000000006
WARNING: CPU: 1 PID: 28510 at lib/usercopy.c:26 _copy_to_user+0xb5/0xc0 lib/usercopy.c:26