tun: free page on build_skb failure in tun_xdp_one()

tun: free page on build_skb failure in tun_xdp_one()

When build_skb() fails in tun_xdp_one(), the function sets ret to
-ENOMEM and jumps to the out label, which returns without freeing the
page that vhost_net_build_xdp() allocated for the frame. As with the
short-frame rejection path, tun_sendmsg() discards the per-buffer error
and still returns total_len, so vhost_tx_batch() takes the success path
and never frees the page. Each build_skb() failure in a batch leaks one
page-frag chunk.

Free the page before taking the error path, matching the put_page() the
other error exits of tun_xdp_one() already perform.

Fixes: 043d222f93ab ("tuntap: accept an array of XDP buffs through sendmsg()")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Reviewed-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260521163312.1479805-2-bestswngs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>