arm64 pmap: Defer bti lookup

Defer the bti lookup until after page table page allocation is complete.
We sometimes release the pmap lock and sleep during page table page
allocation. Consequently, the result of a bti lookup from before
page table page allocation could be stale when we finally create the
mapping based on it.

Modify pmap_bti_same() to update the prototype PTE at the same time as
checking the address range. This eliminates the need for calling
pmap_pte_bti() in addition to pmap_bti_same(). pmap_bti_same() was
already doing most of the work of pmap_pte_bti().

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45502