mm/slab_common: use helper function is_power_of_2()

Use helper function is_power_of_2() to check if KMALLOC_MIN_SIZE is power
of two. Minor readability improvement.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20220217091609.8214-1-linmiaohe@huawei.com