Weaken assertions in pmap_l1_to_l2() and pmap_l2_to_l3().

[markj]

Summary

Avoid looking at the valid bit in the PTE when asserting that we are not
traversing through a superpage. These traversal functions are used by
pmap_kextract(), which locklessly looks up an address, and
pmap_update_entry() will clear an L2 entry's valid bit when promoting or
demoting. L<n>_TABLE includes ATTR_DESCR_VALID, so the assertion could
incorrectly fire when looking up a kernel address involved in concurrent
L2 promotion.

Reported by: mmel
MFC with: r365879

[mmel]

tested on a problematic boards - everything works

[kib]

So could you only weaken it for va >= KVA start ?

[alc]

I'd suggest that you add a comment explaining why the assertion has to be written this way.

[markj]

Add comments.

[markj]

Assert that ATTR_DESCR_VALID is set if va < VM_MAX_USER_ADDRESS.

[markj]

Committed in r365976, I omitted the phabricator link by accident.