Restore nofaulting operations after r352807

[kib]

Summary

The TDP_NOFAULTING flag should be checked in vm_fault(), not in vm_fault_trap(). Otherwise kernel accesses to userspace like vn_io_fault() enter vm locking.

Reported by: pho

[alc]

sys/vm/vm_fault.c

561

This is now only used under KTRACE. I suspect that using curthread directly in those two places will be cheaper than using a callee-saves register for td.

[kib]

Stop using td in vm_fault_trap(), use curthread.