selftests/xsk: Rename xsk_xdp_metadata.h to xsk_xdp_common.h

Rename the header file to a generic name so that it can be used by all
future XDP programs. Ensure that the xsk_xdp_common.h header file includes
include guards.

Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20230927135241.2287547-3-tushar.vyavahare@intel.com