[SCTP]: Remove redundant wrapper functions.

sctp_datamsg_free and sctp_datamsg_track are just aliases for
sctp_datamsg_put and sctp_chunk_hold, respectively.

Saves 32 Bytes on x86.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>