samples/bpf: fix xdpsock l2fwd tx for unaligned mode

Preserve the offset of the address of the received descriptor, and include
it in the address set for the tx descriptor, so the kernel can correctly
locate the start of the packet data.

Fixes: 03895e63 ("samples/bpf: add buffer recycling for unaligned chunks to xdpsock")
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>