nfsd: fix potential UAF in nfsd4_cb_getattr_release

[ Upstream commit 1116e0e372eb16dd907ec571ce5d4af325c55c10 ]

Once we drop the delegation reference, the fields embedded in it are no
longer safe to access. Do that last.

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>