Remove splassert() and turn splassert(IPL_NONE)->assertwaitok().

This removes the remaining splassert() calls, also killing the
unintuitive IPL_NONE semantics for it, every path that used to
splassert(IPL_NONE) wanted to know it is not in an interrupt handler,
I've tightning this restriction by replacing it to assertwaitok(), which
also makes sure you're not holding mutexes.
We don't need IPL_CRIT anymore.

All levels were converted already.