illumos gate

allocb(9f) documents priority, but marks it as unused. Today, allocb() always allocates with KM_NOSLEEP, which will go to some effort to reclaim space if none is immediately available. As seen in #1452, that may not be optimal, especially in interrupt contexts like IP forwarding. #1452 uses KM_NORMALPRI, and it may be something we wish to consider. Making priority mean something would require a fair amount of testing.

If priority truly doesn't matter and we're fine, perhaps we should eliminate it as a parameter. The question of whether or not to use KM_NORMALPRI as well remains open in this case.

If we choose to make priority matter, there are three values BPRI_{LO,MED,HI}. LO should add KM_NORMALPRI to the allocation, HI should not, and MED is a tossup (could even be random?!?).