tomoyo: replace tomoyo_round2() with kmalloc_size_roundup()

It seems tomoyo has had its own implementation of what
kmalloc_size_roundup() does today. Remove the function tomoyo_round2()
and replace it with kmalloc_size_roundup(). It provides more accurate
results and doesn't contain a while loop.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>