iommu_gas: Change find_space lower search order

iommu_gas_lowermatch looks right, then left, then right again in its
search for free space. Change to a more straightforward last-fit
search that touches fewer tree nodes and improves performance.

Reported by: wxzhu@rice.edu
Reviewed by: alc, kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D35439