kern_fail: Stop checking for failures from fp_malloc(M_WAITOK)

fp_malloc is defined as a macro that redirects to malloc.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45852

(cherry picked from commit 6a2a385507c79abaa9db9eabfdd827362f3dc7ed)
(cherry picked from commit fbaf4806511522cdcac7eda3bd5fbcc4b2e2f15c)