sctp: implement SIOCINQ ioctl() (take 3)

This simple patch copies the current approach for SIOCINQ ioctl() from DCCP
into SCTP so that the userland code working with SCTP can use a similar
interface across different protocols to know how much space to allocate for
a buffer.

Signed-off-by: Diego Elio Pettenò <flameeyes@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>