Comments in file include/linux/shrinker.h says that
`count_objects` of `struct shrinker` should return SHRINK_EMPTY
when there are no objects to free.
If there are no objects to free, it should return SHRINK_EMPTY,
while 0 is returned in cases of the number of freeable items cannot
be determined or shrinker should skip this cache for this time
(e.g., their number is below shrinkable limit).
Signed-off-by: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx>