vm_addr_ok: add power2 invariant check

With INVARIANTS defined, have vm_addr_align_ok and vm_addr_bound_ok
panic when passed an alignment/boundary parameter that is not a power
of two.

Reviewed by: alc
Suggested by: kib, se
Differential Revision: https://reviews.freebsd.org/D33725

(cherry picked from commit ae13829ddce0b5fbf12f2b240a26414b41def8ba)