More checks for PG_ZERO pages

[kib]

Summary

vm_page_free_prep(): convert PG_ZERO zeroed page check to use sf_buf

Make the check MI by allocating sf_buf in non-blockable manner.  For
DMAP arches, this should be nop since sf_buf allocation cannot fail.
For non-DMAP arches, we get the checks activated unless there is serious
sf_buf pressure, which typically should be not.

The context for vm_page_free_prep() should be ready to block on some VM
mutexes, which should make it reasonable to block on sf_buf list lock.


vm_fault: under DIAGNOSTIC, verify that PG_ZERO page is indeed zeroed