NFSD: Refactor nfsd_setattr()

[ Upstream commit c0aa1913 ]

Move code that will be retried (in a subsequent patch) into a helper
function.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
[ cel: backported to 5.10.y, prior to idmapped mounts ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>