acpi: Ensure that adjacent memory affinity table entries are coalesced

The SRAT may contain multiple distinct entries that together describe a
contiguous region of physical memory. In this case we were not
coalescing the corresponding entries in the memory affinity table, which
led to fragmented phys_avail[] entries. Since r338431 the vm_phys_segs[]
entries derived from phys_avail[] will be coalesced, resulting in a
situation where vm_phys_segs[] entries do not have a covering
phys_avail[] entry. vm_page_startup() will not add such segments to the
physical memory allocator, leaving them unused.

Reported by: Don Morris <dgmorris@earthlink.net>
Reviewed by: kib, vangyzen
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27620