uma_zalloc: assert M_NOWAIT ^ M_WAITOK

The uma_zalloc functions expect exactly one of [M_NOWAIT, M_WAITOK].
If neither or both are passed, print an error and a stack dump.
Only do this ten times, to prevent livelock. In the future, after
this exposes enough bad callers, this will be changed to a KASSERT().

Reviewed by: rstone, markj
MFC after: 1 month
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D34452