Fix a callout race condition introduced in TCP timers callouts with r281599.
In TCP timer context, it is not enough to check callout_stop() return value
to decide if a callout is still running or not, previous callout_reset()
return values have also to be checked.

Differential Revision: https://reviews.freebsd.org/D2763
Reviewed by: hiren
Approved by: hiren
MFC after: 1 day
Sponsored by: Verisign, Inc.