Disable errors for -Wnonnull for the kernel for GCC 12.

The USB code and some other places raise false positives when a NULL
pointer is passed to an inlined function along with a separate length
and the compiler can't determine that the separate length of 0
prevents the use of the NULL pointer.

Differential Revision: https://reviews.freebsd.org/D37627

(cherry picked from commit 488c498b099e3f48db4048833d2b0d03d7bed3e1)