LinuxKPI: add default_gfp()

Various new allocation macros can take an optional gfp_t argument.
If the argument is not given we need to set the GFP_KERNEL default.
While this is only internally used and I initialy called it differently,
should this spread elsewhere having the same name as in Linux will be
good.

Sponsored by: The FreeBSD Foundaton
MFC after: 3 days
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D56392