Add memory barrier to bus_dmamap_sync()

On platforms which are fully IO-coherent, the map might be null.
We need to guarantee that all data is observable after the
sync operation is called. Add a memory barrier to ensure that on ARM.

Reviewed by: andrew, kib
Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3012