vm_phys: do not ignore phys_avail[] segments that do not fit completely into vm_phys segments

If phys_avail[] segment only intersect with some vm_phys segment, add
pages from it to the free list that belong to the given vm_phys_seg,
instead of dropping them.

The vm_phys segments are generally result of subdivision of phys_avail
segments, for instance DMA32 or LOWMEM boundaries split them. On
amd64, after UEFI in-place kernel activation (copy_staging disable)
was enabled, we typically have a large phys_avail[] segment below 4G
which crosses LOWMEM (1M) boundary. With the current way of requiring
phys_avail[] fully fit into vm_phys_seg, this memory was ignored.

Reported by: madpilot
Reviewed by: markj
Discussed with: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31958