Pass VM_PROT_EXECUTE to vm_fault for instruction faults.

We need to tell vm_fault the reason for the fault was because we tried to
execute from the memory location. Without this it may return with success
as we only request read-only memory, then we return to the same location
and try to execute from the same memory address. This leads to an infinite
loop raising the same fault and returning to the same invalid location.

MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D18511