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 <[email protected]>
Reviewed-by: Gao Xiang <[email protected]>
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 <[email protected]>

Author: black-desk

fs/erofs/zutil.c

@@ -243,7 +243,7 @@ void erofs_shrinker_unregister(struct super_block *sb)
 static unsigned long erofs_shrink_count(struct shrinker *shrink,
 					struct shrink_control *sc)
 {
-	return atomic_long_read(&erofs_global_shrink_cnt);
+	return atomic_long_read(&erofs_global_shrink_cnt) ?: SHRINK_EMPTY;
 }
 
 static unsigned long erofs_shrink_scan(struct shrinker *shrink,