nfsd: hold reference to delegation when updating it for cb_getattr

[ Upstream commit da05ba23d4c8d3e8a45846b952e53dd76c4b5e36 ]

Once we've dropped the flc_lock, there is nothing that ensures that the
delegation that was found will still be around later. Take a reference
to it while holding the lock and then drop it when we've finished with
the delegation.

Fixes: c5967721

 ("NFSD: handle GETATTR conflict with write delegation")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>