vm_reserv: extract common gap-checking code

vm_reserv_alloc_contig and vm_reserv_alloc_page both have code to
determine whether a new reservation will fit between the predecessor
and successor. Extract this code into a separate function rather than
having it appear twice. Optimize slightly to avoid checking object
size limit when there is a successor, and to avoid checking the upper
bound at all when the size to be allocated is a multiple of
reservation size.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49460