xen/netback: check correct frag when looking for head frag

When I backported 7d5145d8eb2b "xen/netback: don't leak pages on failure in
xen_netbk_tx_check_gop" to 3.0 (where it became f0457844e605) I somehow picked
up an extraneous hunk breaking this.

Reported-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>