xsk: Wrap duplicated code to function

Both allocation paths have exactly the same code responsible for getting
and initializing xskb. Pull it out to common function.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20241007122458.282590-6-maciej.fijalkowski@intel.com