tcp: Two bugs in rack one of which can lead to a panic.

In extensive testing in NF we have found two issues inside
the rack stack.

1) An incorrect offset is being generated by the fast send path when a fast send is initiated on
   the end of the socket buffer and before the fast send runs, the sb_compress macro adds data to the trailing socket.
   This fools the fast send code into thinking the sb offset changed and it miscalculates a "updated offset".
   It should only do that when the mbuf in question got smaller.. i.e. an ack was processed. This can lead to
   a panic deref'ing a NULL mbuf if that packet is ever retransmitted. At the best case it leads to invalid data being
   sent to the client which usually terminates the connection. The fix is to have the proper logic (that is in the rsm fast path)
   to make sure we only update the offset when the mbuf shrinks.
2) The other issue is more bothersome. The timestamp check in rack needs to use the msec timestamp when
   comparing the timestamp echo to now. It was using a microsecond timestamp which ends up giving error
   prone results but causes only small harm in trying to identify which send to use in RTT calculations if its a retransmit.

Reviewed by: tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D32062

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