linux: generic: fallback to default ring size with unusual ring lengths
linux: generic: fallback to default ring size with unusual ring lengths

The ethtool -g command may return a ring length of 1 on some NICs.
Since netmap does not manage these degenerate cases, we fall back
on a default length.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap