Weaken assertions in pmap_l1_to_l2() and pmap_l2_to_l3().

pmap_update_entry() will temporarily clear the valid bit of page table
entries in order to satisfy the arm64 pmap's break-before-make
constraint. pmap_kextract() may operate concurrently on kernel page
table pages, introducing windows where the assertions added in r365879
may fail incorrectly since they implicitly assert that the valid bit is
set. Modify the assertions to handle this.

Reviewed by: andrew, mmel (previous version)
Reviewed by: alc, kib
Reported by: mmel, scottph
MFC with: r365879