net/smc: remove duplicate mutex_unlock

For a failing smc_listen_rdma_finish() smc_listen_decline() is
called. If fallback is possible, the new socket is already enqueued
to be accepted in smc_listen_decline(). Avoid enqueuing a second time
afterwards in this case, otherwise the smc_create_lgr_pending lock
is released twice:
[  373.463976] WARNING: bad unlock balance detected!
[  373.463978] 4.18.0-rc7+ #123 Tainted: G           O
[  373.463979] -------------------------------------
[  373.463980] kworker/1:1/30 is trying to release lock (smc_create_lgr_pending) at:
[  373.463990] [<000003ff801205fc>] smc_listen_work+0x22c/0x5d0 [smc]
[  373.463991] but there are no more locks to release!
[  373.463991]
other info that might help us debug this:
[  373.463993] 2 locks held by kworker/1:1/30:
[  373.463994]  #0: 00000000772cbaed ((wq_completion)"events"){+.+.}, at: process_one_work+0x1ec/0x6b0
[  373.464000]  #1: 000000003ad0894a ((work_completion)(&new_smc->smc_listen_work)){+.+.}, at: process_one_work+0x1ec/0x6b0
[  373.464003]
stack backtrace:
[  373.464005] CPU: 1 PID: 30 Comm: kworker/1:1 Kdump: loaded Tainted: G           O      4.18.0-rc7uschi+ #123
[  373.464007] Hardware name: IBM 2827 H43 738 (LPAR)
[  373.464010] Workqueue: events smc_listen_work [smc]
[  373.464011] Call Trace:
[  373.464015] ([<0000000000114100>] show_stack+0x60/0xd8)
[  373.464019]  [<0000000000a8c9bc>] dump_stack+0x9c/0xd8
[  373.464021]  [<00000000001dcaf8>] print_unlock_imbalance_bug+0xf8/0x108
[  373.464022]  [<00000000001e045c>] lock_release+0x114/0x4f8
[  373.464025]  [<0000000000aa87fa>] __mutex_unlock_slowpath+0x4a/0x300
[  373.464027]  [<000003ff801205fc>] smc_listen_work+0x22c/0x5d0 [smc]
[  373.464029]  [<0000000000197a68>] process_one_work+0x2a8/0x6b0
[  373.464030]  [<0000000000197ec2>] worker_thread+0x52/0x410
[  373.464033]  [<000000000019fd0e>] kthread+0x15e/0x178
[  373.464035]  [<0000000000aaf58a>] kernel_thread_starter+0x6/0xc
[  373.464052]  [<0000000000aaf584>] kernel_thread_starter+0x0/0xc
[  373.464054] INFO: lockdep is turned off.

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>