arm64: Handle an unaligned start in pmap_mask_set_locked()

In pmap_mask_set_locked(), correctly handle a starting address that is
in the middle of an L3C page. The symptoms arising from this error
included assertion failures in pmap_demote_l3c().

Reported by: andrew
Reviewed by: markj
Fixes: fd6cb031f577 "arm64 pmap: Add ATTR_CONTIGUOUS support [Part 1]"
Differential Revision: https://reviews.freebsd.org/D45851