Several bug fixes and robustness improvements for the AP boot page
table allocation.

At the time that mp_bootaddress() is called, phys_avail[] array does
not reflect some memory reservations already done, like kernel
placement. Recent changes to DMAP protection which make kernel text
read-only in DMAP revealed this, where on some machines AP boot page
tables selection appears to intersect with the kernel itself.

Fix this by checking the addresses selected using the same algorithm
as bootaddr_rwx(). Also, try to chomp pages for the page table not
only at the start of the contiguous range, but also at the end. This
should improve robustness when the only suitable range is already
consumed by the kernel.

Reported and tested by: Michael Gmelin <freebsd@grem.de>
Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
Differential revision: https://reviews.freebsd.org/D16907