busdma_iommu: Fine-grained locking for the dmamap's map list

Introduce fine-grained locking on the dmamap's list of map entries,
replacing the use of the domain lock. This is not the most significant
source of lock contention, but it is the easiest to address.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35557