always initialize ni_bufs_head in netmap_if
always initialize ni_bufs_head in netmap_if

ni_bufs_head was not properly initialized when no external buffers were
requestedx and contained the ni_bufs_head from the last request. This
was causing spurious buffer frees when alternating between apps that
used external buffers and apps that did not use them.

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