Fix tcp_output() so that tcpcb is updated in the same manner when an
mbuf allocation fails, as in a case when ip_output() returns error.

To achieve that, move large block of code that updates tcpcb below
the out: label.

This fixes a panic, that requires the following sequence to happen:

1) The SYN was sent to the network, tp->snd_nxt = iss + 1, tp->snd_una = iss
2) The retransmit timeout happened for the SYN we had sent,
   tcp_timer_rexmt() sets tp->snd_nxt = tp->snd_una, and calls tcp_output().
   In tcp_output m_get() fails.
3) Later on the SYN|ACK for the SYN sent in step 1) came,
   tcp_input sets tp->snd_una += 1, which leads to
   tp->snd_una > tp->snd_nxt inconsistency, that later panics in
   socket buffer code.

For reference, this bug fixed in DragonflyBSD repo:

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/1ff9b7d322dc5a26f7173aa8c38ecb79da80e419

Reviewed by:	andre
Tested by:	pho
Sponsored by:	Nginx, Inc.
PR:		kern/177456
Submitted by:	HouYeFei&XiBoLiu <lglion718 163.com>
Remove contractions.
Minor spelling and grammar fixes.