[SCTP]: SCTP_CMD_INIT_FAILED annotations.

argument stored for SCTP_CMD_INIT_FAILED is always __be16
(protocol error).  Introduced new field and accessor for
it (SCTP_PERR()); switched to their use (from SCTP_U32() and
.u32)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>