According to Section D5.10.3 "Maintenance requirements on changing System
register values" of the architecture manual, an isb instruction should be
executed after updating ttbr0_el1 and before invalidating the TLB. The
lack of this instruction in pmap_activate() appears to be the reason why
andrew@ and I have observed an unexpected TLB entry for an invalid PTE on
entry to pmap_enter_quick_locked(). Thus, we should now be able to revert
the workaround committed in r349442.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20904