sctp: improve sctp_pathmtu_adjustment()

Allow the resending of DATA chunks to be controlled by the caller,
which allows retiring sctp_mtu_size_reset() in a separate commit.
Also improve the computaion of the overhead and use 32-bit integers
consistently.
Thanks to Timo Voelker for pointing me to the code.

(cherry picked from commit 2de2ae331be2504d1038b2124e7d44f23aa70405)