Pass malloc flags directly through kevent(2) subroutines.

Some kevent functions have a boolean "waitok" parameter for use when
calling malloc(9). Replace them with the corresponding malloc() flags:
the desired behaviour is known at compile-time, so this eliminates a
couple of conditional branches, and makes the code easier to read.

No functional change intended.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18318