net: sctp: Rename SCTP_XMIT_NAGLE_DELAY to SCTP_XMIT_DELAY

MSG_MORE and 'corking' a socket would require that the transmit of
a data chunk be delayed.
Rename the return value to be less specific.

Signed-off-by: David Laight <david.laight@aculab.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>