vm_phys: fix seg->end test in alloc_seg_contig

In vm_phys_alloc_seg_contig, in allocating multiple memory blocks for
a huge allocation, ensure that the end of the allocated range does not
exceed the upper segment limit.

Reorder a couple of checks to improve code layout.

Reviewed by: alc
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33870