o Call vm_map_growstack() from vm_fault() if vm_map_lookup() has failed

 o Call vm_map_growstack() from vm_fault() if vm_map_lookup() has failed
   due to conditions that suggest the possible need for stack growth.
   This has two beneficial effects: (1) we can
   now remove calls to vm_map_growstack() from the MD trap handlers and (2)
   simple page faults are faster because we no longer unnecessarily perform
   vm_map_growstack() on every page fault.
 o Remove vm_map_growstack() from the i386's trap_pfault().
 o Remove the acquisition and release of Giant from i386's trap_pfault().
   (vm_fault() still acquires it.)