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.)
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