Reinstate locks around uvm_fault calls from upageflttrap and kpageflttrap

Reinstate locks around uvm_fault calls from upageflttrap and kpageflttrap
on i386.  This is a backout of revision 1.152.

Kernel crash with messages printed concurrently from multiple CPUs
occasionally seen during ports build:

"WARNING: SPL NOT LOWERED ON TRAP EXIT"

and these panics

ddb{1}> sh panic
cpu1: uvm_fault(0xd470a0a0, 0xcf9b7000, 0, 1) -> e
cpu3: kernel diagnostic assertion "!_kernel_lock_held()" failed: file "/usr/src/sys/uvm/uvm_map.c", line 2707