tap: free page on error paths in tap_get_user_xdp()

tap: free page on error paths in tap_get_user_xdp()

tap_get_user_xdp() rejects a frame shorter than ETH_HLEN with -EINVAL,
and returns -ENOMEM when build_skb() fails. Both paths jump to the err
label without freeing the page that vhost_net_build_xdp() allocated for
the frame. tap_sendmsg() discards the per-buffer return value and always
returns 0, so vhost_tx_batch() takes the success path and never frees
the page; each rejected frame in a batch leaks one page-frag chunk.

Free the page on both error paths, before the skb is built. This is the
tap counterpart of the same leak in tun_xdp_one().

Fixes: 0efac27791ee ("tap: accept an array of XDP buffs through sendmsg()")
Fixes: ed7f2afdd0e0 ("tap: add missing verification for short frame")
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/20260521163230.1478627-2-bestswngs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>