netmap: import changes from upstream

• make sure rings are disabled during resets
• introduce netmap_update_hostrings_mode(), with support for multiple host rings
• 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.
• check na validitity under lock on detach
• netmap_mem: fix leak on error path
• nm_dispatch: fix compilation on Raspberry Pi

MFC after: 2 weeks