LinuxKPI: move hex2bin() from kernel.h to new hex.h

New Linux v7.0 drivers include hex.h. Rather than adding a dummy
header, migrate the kernel.h hex2bin() into hex.h, where it belongs.
Care needs to be taken as the _h2b() helper function is still used by
other bits in kernel.h.

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste, dumbbell
Differential Revision: https://reviews.freebsd.org/D56391

(cherry picked from commit 98297ff3cfbb42df86fa7af51a2740d5aa7236eb)