tcp: improve BBLog for output events when using the FreeBSD stack

Put the return value of ip_output()/ip6_output in the output event
instead of adding another one in case of an error. This improves
consistency with other similar places.

Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D36085

(cherry picked from commit bd30a1216e03e14cfaa7e91b1fc72ad76e9cd240)