Add an in kernel nbd-server implementation

This provides a barebones NBD server in the kernel.  The userspace server will
still be responsible for providing exports and negotiating connections with the
nbd client.  Then it will pass the kernel the device that we are exporting and
the socket for the connection that was opened and negotiated.  From there the
kernel will handle all of the commands directly and handle cleanup.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>