pmap_pv_alloc vs. PMAP_CANFAIL

[Masao Uebayashi <uebayasi%gmail.com@localhost>]

pmap_enter() calls pmap_enter_pv() if a given address has page caches
(== struct vm_page * objects).  pmap_enter_pv() in turn calls
pmap_pv_alloc(), which allocates a struct pv_entry * object from its
dedicated pool.

Now pmap_pv_alloc() always call pool_get() with PR_NOWAIT.  If
pool_get() fails, it just panics.  pmap_enter(9) should not panic but
return failure status to caller if it's given PMAP_CANFAIL.

Masao