- Small cleanups to the Giant handling in trap(). - Only release Giant in trap() if we locked it, otherwise we could release   Giant in a kernel trap if we didn't get it for a page fault and the   previous frame had grabbed the lock. - Only get Giant for !MP safe syscalls.
- Small cleanups to the Giant handling in trap().
- Only release Giant in trap() if we locked it, otherwise we could release
  Giant in a kernel trap if we didn't get it for a page fault and the
  previous frame had grabbed the lock.
- Only get Giant for !MP safe syscalls.

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