Eliminate typically pointless calls to vm_fault_prefault() on soft, copy-
on-write faults. On a page fault, when we call vm_fault_prefault(), it
probes the pmap and the shadow chain of vm objects to see if there are
opportunities to create read and/or execute-only mappings to neighoring
pages. For example, in the case of hard faults, such effort typically pays
off, that is, mappings are created that eliminate future soft page faults.
However, in the the case of soft, copy-on-write faults, the effort very
rarely pays off. (See the review for some specific data.)

Reviewed by: kib, markj
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D17367