Check for wrap-around in vm_phys_alloc_seg_contig().

It is possible to provide insane values for size in contigmalloc(9)
request, which usually not reaches the phys allocator due to failing
KVA allocation.  But with the forthcoming 4/4 i386, where 32bit
architecture has almost 4G KVA, contigmalloc(1G) is not unreasonable
outright and KVA might be available sometimes.

Then, the calculation of pa_end could wrap around, depending on the
physical address, and the checks in vm_phys_alloc_seg_contig() would
pass while the iteration in the loop after the 'done' label goes out
of the vm_page_array bounds.

Fix it by detecting the wrap.

Reported and tested by:	pho
Reviewed by:	alc, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D14767

sys/vm/vm_phys.c

@@ -1189,6 +1189,8 @@ vm_phys_alloc_seg_contig(struct vm_phys_seg *seg, u_long npages,
 					 */
 					pa = VM_PAGE_TO_PHYS(m_ret);
 					pa_end = pa + size;
+					if (pa_end < pa)
+						continue;
 					for (;;) {
 						pa += 1 << (PAGE_SHIFT +
 						    VM_NFREEORDER - 1);