net/ipv6: fix lock imbalance in ip6_route_del()

WARNING: lock held when returning to user space!
4.17.0-rc3+ #37 Not tainted

syz-executor1/27662 is leaving the kernel with locks still held!
1 lock held by syz-executor1/27662:
 #0: 00000000f661aee7 (rcu_read_lock){....}, at: ip6_route_del+0xea/0x13f0 net/ipv6/route.c:3206
BUG: scheduling while atomic: syz-executor1/27662/0x00000002
INFO: lockdep is turned off.
Modules linked in:
Kernel panic - not syncing: scheduling while atomic

CPU: 1 PID: 27662 Comm: syz-executor1 Not tainted 4.17.0-rc3+ #37
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1b9/0x294 lib/dump_stack.c:113
 panic+0x22f/0x4de kernel/panic.c:184
 __schedule_bug.cold.85+0xdf/0xdf kernel/sched/core.c:3290
 schedule_debug kernel/sched/core.c:3307 [inline]
 __schedule+0x139e/0x1e30 kernel/sched/core.c:3412
 schedule+0xef/0x430 kernel/sched/core.c:3549
 exit_to_usermode_loop+0x220/0x310 arch/x86/entry/common.c:152
 prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:265 [inline]
 do_syscall_64+0x6ac/0x800 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x455979
RSP: 002b:00007fbf4051dc68 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: 0000000000000000 RBX: 00007fbf4051e6d4 RCX: 0000000000455979
RDX: 00000000200001c0 RSI: 000000000000890c RDI: 0000000000000013
RBP: 000000000072bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000000003c8 R14: 00000000006f9b60 R15: 0000000000000000
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..

Fixes: 23fb93a4 ("net/ipv6: Cleanup exception and cache route handling")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Ahern <dsahern@gmail.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -3224,8 +3224,10 @@ static int ip6_route_del(struct fib6_config *cfg,
 							      &cfg->fc_src);
 				if (rt_cache) {
 					rc = ip6_del_cached_rt(rt_cache, cfg);
-					if (rc != -ESRCH)
+					if (rc != -ESRCH) {
+						rcu_read_unlock();
 						return rc;
+					}
 				}
 				continue;
 			}