To test to see if a free space is big enough compare the required
length to the difference of the two offsets that define the gap, to
avoid overflow, rather that adding the length to an offset and
comparing that to another offset.

This addresses an overflow issue reported by Peter Holm on i386.

Reported by: pho
Tested by: pho
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D20594