net: Move the comment about unsettable socket-level options to default clause and update its reference.

We implement the SO_SNDLOWAT etc not to be settable and return
ENOPROTOOPT per 1003.1g 7.  Move the comment to appropriate
position and update the reference.
Signed-off-by: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>