ceph_wait_on_conflict_unlink(): grab reference before dropping ->d_lock

[ Upstream commit dc32464a5fe4946fe1a4d8f8e29961dc411933c5 ]

Use of dget() after we'd dropped ->d_lock is too late - dentry might
be gone by that point.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>