Merge branch 'sctp-MSG_MORE'

Xin Long says:

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sctp: support MSG_MORE flag when sending msg

This patch is to add support for MSG_MORE on sctp. Patch 1/2 is an
improvement ahead of patch 2/2 to solve the close block problem
mentioned in https://patchwork.ozlabs.org/patch/372404/

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Signed-off-by: David S. Miller <davem@davemloft.net>