MFS12 r342378: Fix a regression in the TCP handling of received segments.

When receiving TCP segments the stack protects itself by limiting
the resources allocated for a TCP connections. This patch adds
an exception to these limitations for the TCP segement which is the next
expected in-sequence segment. Without this patch, TCP connections
may stall and finally fail in some cases of packet loss.

Approved by: so
Security: FreeBSD-EN-19:02.tcp