malloc(9): assert wait flags.

The check introduced in d3c11994e1e3 was bogus, combining a non-atomic
“once” flag with an equally non-thread-safe ppsratecheck. Rather than
fix it, just assert what it attempts to enforce: that the malloc flags
must include exactly one of M_WAITOK and M_NOWAIT.

Fixes: d3c11994e1e3de7445305abd0d41dce2b8d3e6dc
Sponsored by: Klara, Inc.
Reviewed by: olce, kevans
Differential Revision: https://reviews.freebsd.org/D47309