kernel: Move us to using M_NOWAIT and M_WAITOK for mbuf functions.

kernel: Move us to using M_NOWAIT and M_WAITOK for mbuf functions.

The main reason is that our having to use the MB_WAIT and MB_DONTWAIT
flags was a recurring issue when porting drivers from FreeBSD because
it tended to get forgotten and the code would compile anyway with the
wrong constants. And since MB_WAIT and MB_DONTWAIT ended up as ocflags
for an objcache_get() or objcache_reclaimlist call (which use M_WAITOK
and M_NOWAIT), it was just one big converting back and forth with some
sanitization in between.

This commit allows M_* again for the mbuf functions and keeps the
sanitizing as it was before: when M_WAITOK is among the passed flags,
objcache functions will be called with M_WAITOK and when it is absent,
they will be called with M_NOWAIT. All other flags are scrubbed by the
MB_OCFLAG() macro which does the same as the former MBTOM().

Approved-by: dillon