[SCTP]: Fix warning

An alternate solution would be to make the digest a pointer, allocate
it in sctp_endpoint_init() and free it in sctp_endpoint_destroy().

I guess I should have originally done it this way...

  CC [M]  net/sctp/sm_make_chunk.o
net/sctp/sm_make_chunk.c: In function 'sctp_unpack_cookie':
net/sctp/sm_make_chunk.c:1358: warning: initialization discards qualifiers from pointer target type

The reason is that sctp_unpack_cookie() takes a const struct
sctp_endpoint and modifies the digest in it (digest being embedded in
the struct, not a pointer).  Make digest a pointer to fix this
warning.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Acked-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>