Revision 211333

Fix the interaction between 'ICMP fragmentation needed' MTU updates,
path MTU discovery and the tcp_minmss limiter for very small MTU's.

When the MTU suggested by the gateway via ICMP, or if there isn't
any the next smaller step from ip_next_mtu(), is lower than the
floor enforced by net.inet.tcp.minmss (default 216) the value is
ignored and the default MSS (512) is used instead.  However the
DF flag in the IP header is still set in tcp_output() preventing
fragmentation by the gateway.

Fix this by using tcp_minmss as the MSS and clear the DF flag if
the suggested MTU is too low.  This turns off path MTU dissovery
for the remainder of the session and allows fragmentation to be
done by the gateway.

Only MTU's smaller than 256 are affected.  The smallest official
MTU specified is for AX.25 packet radio at 256 octets.

PR:		kern/146628
Tested by:	Matthew Luckie <mjl-at-luckie org nz>
MFC after:	1 week