hammer2 - Move write thread from hmp to pmp

* Move the write thread from struct hammer2_mount to struct hammer2_pfsmount.
  Logical buffers for files are associated with the PFS, not the HMP.  A PFS
  can be backed by multiple HMPs.
kernel - Fix a SMP race in the 64-bit pmap code (2)

* Last commit had an extra vm_page_spin_unlock() call, removing.

* Fix an improper spin_unlock() which was supposed to be a
  spin_unlock_shared().  This appears to be an old bug.

Reported-by: profmakx, ftigeot