TCP RACK: fix TCP fast open

Do not jump to a place in the code, which requires several variables
to be set (segsize, minseg, idle, len, sb_offset), which is not true.
To avoid using these variables, start the HPTS timer explicitly.
This fix only applies to the client side using TCP fast open.

Approved by: rrs
CID: 1523766
CID: 1523770
CID: 1523786
CID: 1523801
CID: 1523809
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48322