rxrpc: Fix locking issues in rxrpc_put_peer_locked()

Now that rxrpc_put_local() may call kthread_stop(), it can't be called
under spinlock as it might sleep.  This can cause a problem in the peer
keepalive code in rxrpc as it tries to avoid dropping the peer_hash_lock
from the point it needs to re-add peer->keepalive_link to going round the
loop again in rxrpc_peer_keepalive_dispatch().

Fix this by just dropping the lock when we don't need it and accepting that
we'll have to take it again.  This code is only called about every 20s for
each peer, so not very often.

This allows rxrpc_put_peer_unlocked() to be removed also.

If triggered, this bug produces an oops like the following, as reproduced
by a syzbot reproducer for a different oops[1]:

BUG: sleeping function called from invalid context at kernel/sched/completion.c:101
...
RCU nest depth: 0, expected: 0
3 locks held by kworker/u9:0/50:
 #0: ffff88810e74a138 ((wq_completion)krxrpcd){+.+.}-{0:0}, at: process_one_work+0x294/0x636
 #1: ffff8881013a7e20 ((work_completion)(&rxnet->peer_keepalive_work)){+.+.}-{0:0}, at: process_one_work+0x294/0x636
 #2: ffff88817d366390 (&rxnet->peer_hash_lock){+.+.}-{2:2}, at: rxrpc_peer_keepalive_dispatch+0x2bd/0x35f
...
Call Trace:
 <TASK>
 dump_stack_lvl+0x4c/0x5f
 __might_resched+0x2cf/0x2f2
 __wait_for_common+0x87/0x1e8
 kthread_stop+0x14d/0x255
 rxrpc_peer_keepalive_dispatch+0x333/0x35f
 rxrpc_peer_keepalive_worker+0x2e9/0x449
 process_one_work+0x3c1/0x636
 worker_thread+0x25f/0x359
 kthread+0x1a6/0x1b5
 ret_from_fork+0x1f/0x30

Fixes: a275da62 ("rxrpc: Create a per-local endpoint receive queue and I/O thread")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/0000000000002b4a9f05ef2b616f@google.com/

 [1]
Signed-off-by: David S. Miller <davem@davemloft.net>