nfsd: filecache: drop the list_lru lock during lock gc scans

nfsd: filecache: drop the list_lru lock during lock gc scans

Under a high NFSv3 load with lots of different files being accessed,
the LRU list of garbage-collectable files can become quite long.

Asking list_lru_scan_node() to scan the whole list can result in a long
period during which a spinlock is held, blocking the addition of new LRU
items.

So ask list_lru_scan_node() to scan only a few entries at a time, and
repeat until the scan is complete.

If the shrinker runs between two consecutive calls of
list_lru_scan_node() it could invalidate the "remaining" counter which
could lead to premature freeing.  So add a spinlock to avoid that.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>