Revert r265338, r271089 and r271123 as those changes do not handle
non-inline urgent data and introduce an mbuf exhaustion attack vector
similar to FreeBSD-SA-15:15.tcp, but not requiring VNETs.

Address the issue described in FreeBSD-SA-15:15.tcp.

Reviewed by: glebius
Approved by: so
Approved by: jmallett (mentor)
Security: FreeBSD-SA-15:15.tcp
Sponsored by: Norse Corp, Inc.