xsk: respect the offsets when copying frags

xsk: respect the offsets when copying frags

In commit 560d958c6c68 ("xsk: add generic XSk &xdp_buff -> skb
conversion"), we introduce a helper to convert zerocopy xdp_buff to skb.
However, in the frag copy, we mistakenly ignore the frag's offset. This
commit adds the missing offset when copying frags in
xdp_copy_frags_from_zc(). This function is not used anywhere so no
backport is needed.

Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Link: https://patch.msgid.link/20250426081220.40689-2-minhquangbui99@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>