mds: remove waiting lock before merging with neighbours

CephFS currently deadlocks under CTDB's ping_pong POSIX locking test
when run concurrently on multiple nodes.
The deadlock is caused by failed removal of a waiting_locks entry when
the waiting lock is merged with an existing lock, e.g:

Initial MDS state (two clients, same file):
held_locks -- start: 0, length: 1, client: 4116, pid: 7899, type: 2
	      start: 2, length: 1, client: 4110, pid: 40767, type: 2
waiting_locks -- start: 1, length: 1, client: 4116, pid: 7899, type: 2

Waiting lock entry 4116@1:1 fires:
handle_client_file_setlock: start: 1, length: 1,
			    client: 4116, pid: 7899, type: 2

MDS state after lock is obtained:
held_locks -- start: 0, length: 2, client: 4116, pid: 7899, type: 2
	      start: 2, length: 1, client: 4110, pid: 40767, type: 2
waiting_locks -- start: 1, length: 1, client: 4116, pid: 7899, type: 2

Note that the waiting 4116@1:1 lock entry is merged with the existing
4116@0:1 held lock to become a 4116@0:2 held lock. However, the now
handled 4116@1:1 waiting_locks entry remains.

When handling a lock request, the MDS calls adjust_locks() to merge
the new lock with available neighbours. If the new lock is merged,
then the waiting_locks entry is not located in the subsequent
remove_waiting() call because adjust_locks changed the new lock to
include the old locks.
This fix ensures that the waiting_locks entry is removed prior to
modification during merge.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit 476e4902907dfadb3709ba820453299ececf990b)