mm/slab_common: move declaration of __ksize() to mm/slab.h

__ksize() is only called by KASAN. Remove export symbol and move
declaration to mm/slab.h as we don't want to grow its callers.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>