xsk: Remove unnecessary member in xdp_umem

Remove the unnecessary member of address in struct xdp_umem as it is
only used during the umem registration. No need to carry this around
as it is not used during run-time nor when unregistering the umem.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Link: https://lore.kernel.org/bpf/1588599232-24897-3-git-send-email-magnus.karlsson@intel.com