erofs: return SHRINK_EMPTY if no objects to free

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@uniontech.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/149E6E64B5B6B5E8+20250116083303.199817-1-chenlinxuan@uniontech.com


[ Gao Xiang: should have no impact since it's not memcg-aware. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>