sctp: improve some variables in sctp_sendmsg

This patch mostly is to:

  - rename sinfo_flags as sflags, to make the indents look better, and
    also keep consistent with other sctp_sendmsg_xx functions.

  - replace new_asoc with bool new, no need to define a pointer here,
    as if new_asoc is set, it must be asoc.

  - rename the 'out_nounlock:' as 'out', shorter and nicer.

  - remove associd, only one place is using it now, just use
    sinfo->sinfo_assoc_id directly.

  - remove 'cmsgs' initialization in sctp_sendmsg, as it will be done
    in sctp_sendmsg_parse.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>