To check if txsync can be skipped, it is necessary to look for
unseen TX space. However, this means comparing ring->cur
against ring->tail, rather than ring->head against ring->tail
(like nm_ring_empty() does).
This change also adds some more comments to explain the optimization
performed at the beginning of netmap_poll().
MFC after: 3 days
Sponsored by: Sunny Valley Networks
58e185425a