Achieve two goals at once: (1) Avoid an unnecessary broadcast TLB
invalidation in pmap_remove_all(). (2) Prevent an "invalid ASID" assertion
failure in pmap_remove_all().

The architecture definition specifies that the TLB will not cache mappings
that don't have the "AF" bit set, so pmap_remove_all() needn't issue a TLB
invalidation for mappings that don't have the "AF" bit set.

We allocate ASIDs lazily. Specifically, we don't allocate an ASID for a
pmap until we are activating it. Now, consider what happens on a fork().
Before we activate the child's pmap, we use pmap_copy() to copy mappings
from the parent's pmap to the child's. These new mappings have their "AF"
bits cleared. Suppose that the page daemon decides to reclaim a page that
underlies one of these new mappings. Previously, the pmap_invalidate_page()
performed by pmap_remove_all() on a mapping in the child's pmap would fail
an assertion because that pmap hasn't yet been assigned an ASID. However,
we don't need to issue a TLB invalidation for such mappings because they
can't possibly be cached in the TLB.

Reported by: bob prohaska <fbsd@www.zefox.net>
Reviewed by: markj
MFC after: 1 week
X-MFC-before: r354286
Differential Revision: https://reviews.freebsd.org/D22388