NFSD: Add nfsd_file_lru_dispose_list() helper

[ Upstream commit 0bac5a26 ]

Refactor the invariant part of nfsd_file_lru_walk_list() into a
separate helper function.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>