Merge branch 'sctp-refactor-MTU-handling'

Marcelo Ricardo Leitner says:

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sctp: refactor MTU handling

Currently MTU handling is spread over SCTP stack. There are multiple
places doing same/similar calculations and updating them is error prone
as one spot can easily be left out.

This patchset converges it into a more concise and consistent code. In
general, it moves MTU handling from functions with bigger objectives,
such as sctp_assoc_add_peer(), to specific functions.

It's also a preparation for the next patchset, which removes the
duplication between sctp_make_op_error_space and
sctp_make_op_error_fixed and relies on sctp_mtu_payload introduced here.

More details on each patch.
====================

Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>