malloc_aligned: roundup allocation size up to next power of two

to make it use the right aligned zone.

Reported by: melifaro
Reviewed by: alc, markj (previous version)
Discussed with: jrtc27
Tested by: pho (previous version)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28219