[SCTP] Don't disable PMTU discovery when mtu is small

Right now, when we receive a mtu estimate smaller then minim
threshold in the ICMP message, we disable the path mtu discovery
on the transport.  This leads to the never increasing sctp fragmentation
point even when the real path mtu has increased.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>