amd64 pmap: handle fictitious mappigns with addresses beyond pv_table

There are provisions to do it already with pv_dummy, but new locking code
did not account for it. Previous one did not have the problem because
it hashed the address into the lock array.

While here annotate common vars with read_mostly and exclusive_cache_line.

Reported by: Thomas Laus
Tesetd by: jkim, Thomas Laus
Fixes: r353149 ("amd64 pmap: implement per-superpage locks")
Sponsored by: The FreeBSD Foundation