kernel - Fix a SMP race in the 64-bit pmap code

kernel - Fix a SMP race in the 64-bit pmap code

* Some of the pmap code was assuming that pv_entry's would remain stable
  under circumstance where they might not actually remain stable.

  Specifically, when the vm_page spinlock is held while issuing a
  pv_hold_try() call on the pv, the pv is not necessarily stable
  and its pmap and pindex must be double-checked after a successful
  lock is acquired.  This case occurs in two places in the code.

* Do this check in all cases (when the pmap spinlock OR the vm_page spin
  lock is used to access a pv_entry), just to be sure that we have covered
  all the bases.

* This SMP race is virtually impossible to reproduce on 8-thread boxes
  but appears to be easy to reproduce on monster (48-way opteron).

* As of this commit it is not 100% clear if the patch fixes the assertion
  seen, but it probably fixes some things.

  panic: bad *ptep 000000078592b425 sva 00007ffffffff000 pte_pv NULL

Reported-by: ftigeot