sctp: introduce sctp_dst_mtu

sctp: introduce sctp_dst_mtu

BugLink: http://bugs.launchpad.net/bugs/1812229
[ Upstream commit 6ff0f871c20ec1769a481edca86f23c76b2b06d3 ]

Which makes sure that the MTU respects the minimum value of
SCTP_DEFAULT_MINSEGMENT and that it is correctly aligned.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>