xsk: Add support for recvmsg()

Add support for non-blocking recvmsg() to XDP sockets. Previously,
only sendmsg() was supported by XDP socket. Now, for symmetry and the
upcoming busy-polling support, recvmsg() is added.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20201130185205.196029-4-bjorn.topel@gmail.com