stand/kmem_zalloc: panic when a M_WAITOK allocation fails

Malloc() might return NULL, in which case we will panic with a NULL
pointer deref. Make it panic when the allocation fails to preserve the
postcondtion that we never return a non-NULL value.

Reviewed by: tsoome
PR: 249859
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31106