mm/slub: Remove the unneeded result variable

Return the value from attribute->store(s, buf, len) and
attribute->show(s, buf) directly instead of storing it in
another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>