Merge branch 'sctp-gso-frags-from-chunk'

Marcelo Ricardo Leitner says:

====================
sctp: allow GSO frags to access the chunk too

Patchset is named after the most important fix in it. First two patches
are preparing the grounds for the 3rd patch.

After the 3rd, they are not strictly logically related to the patchset,
but I kept them together as they depend on each other.

More details on patch changelogs.
====================

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