if_awg: avoid hole in the rx ring buffer when mbuf allocation fails

Use a spare dma map when attempting to map a new mbuf on the rx path.
If the mbuf allocation fails or the dma map loading for the new mbuf fails just reuse the old mbuf
and increase the drop counter.

Submitted by: Guy Yur <guyyur@gmail.com>
Differential Revision: https://reviews.freebsd.org/D12538