tcp: Don't prematurely drop receiving-only connections

If the connection was persistent and receiving-only, several (12)
sporadic device insufficient buffers would cause the connection be
dropped prematurely:
Upon ENOBUFS in tcp_output() for an ACK, retransmission timer is
started. No one will stop this retransmission timer for receiving-
only connection, so the retransmission timer promises to expire and
t_rxtshift is promised to be increased. And t_rxtshift will not be
reset to 0, since no RTT measurement will be done for receiving-only
connection. If this receiving-only connection lived long enough
(e.g. >350sec, given the RTO starts from 200ms), and it suffered 12
sporadic device insufficient buffers, i.e. t_rxtshift >= 12, this
receiving-only connection would be dropped prematurely by the
retransmission timer.

We now assert that for data segments, SYNs or FINs either rexmit or
persist timer was wired upon ENOBUFS. And don't set rexmit timer
for other cases, i.e. ENOBUFS upon ACKs.

Discussed with: lstewart, hiren, jtl, Mike Karels
MFC after: 3 weeks
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5872