[PATCH] ip_output: account for fraggap when checking to add trailer_len

During other work I noticed that ip_append_data() seemed to be forgetting to
include the frag gap in its calculation of a fragment that consumes the rest of
the payload.  Herbert confirmed that this was a bug that snuck in during a
previous rework.

Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>