ionic: add helpers for accessing buffer info

These helpers clean up some of the code around DMA mapping
and other buffer references, and will be used in the next
few patches for the XDP support.

Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>