Fix a race in pmap_emulate_modified().

pmap_emulate_modify() was assuming that no changes to the pmap could
take place between the TLB signaling the fault and
pmap_emulate_modify()'s acquisition of the pmap lock, but that's clearly
not even true in the uniprocessor case, nevermind the SMP case.

Submitted by: Nathaniel Filardo <nwf20@cl.cam.ac.uk>
Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24523