sctp: add sctp_info dump api for sctp_diag

sctp_diag will dump some important details of sctp's assoc or ep, we use
sctp_info to describe them,  sctp_get_sctp_info to get them, and export
it to sctp_diag.ko.

v2->v3:
- we will not use list_for_each_safe in sctp_get_sctp_info, cause
  all the callers of it will use lock_sock.

- fix the holes in struct sctp_info with __reserved* field.
  because sctp_diag is a new feature, and sctp_info is just for now,
  it may be changed in the future.

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