vm_phys: avoid waste in multipage allocation

vm_phys: avoid waste in multipage allocation

In vm_phys_alloc_contig, for an allocation bigger than the size of any
buddy queue free block, avoid examining any maximum-size free block
more than twice, by only starting to consider a sequence of adjacent
max-blocks starting at a max-block that does not follow another
max-block.  If that first max-block follows adjacent blocks of smaller
size, and if together they provide enough memory to reduce by one the
number of max-blocks required for this allocation, use them as part of
this allocation.

Reviewed by:	markj
Tested by:	pho
Discussed with:	alc
Differential Revision:	https://reviews.freebsd.org/D34815