iommu: Shrink the iommu map entry structure

Eliminate the unroll_entry field from struct iommu_map_entry, shrinking
the struct by 16 bytes on 64-bit architectures.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35769

(cherry picked from commit db0110a536bf70c1ff55f3b3f46a0b5a9af46058)