iommu_gas: make iommu_gas_lowermatch non-recursive

Change the recursive implementation to one that uses parent pointers
to walk back up the rb-tree, to slightly improve performance.

Reviewed by: alc, kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D35486

(cherry picked from commit f979ad00306508f0c9fc925ec05b2413b70ab5f1)