xsk: add Tx queue setup and mmap support

Another setsockopt (XDP_TX_QUEUE) is added to let the process allocate
a queue, where the user process can pass frames to be transmitted by
the kernel.

The mmapping of the queue is done using the XDP_PGOFF_TX_QUEUE offset.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>