In the pmap_set_pg() function, which enables the global bit on the
ptes mapping the kernel on CPUs where global TLB entries are
supported, revert to flushing only non-global entries, i.e. to the
pre-r291688 state. There is no need to flush global TLB entries,
since only global entries created during the previous iterations of
the loop could exist at this moment.

Submitted by: alc
Differential revision: https://reviews.freebsd.org/D4368