socket-util: Use correct address family in set_dscp(), instead of guessing.

socket-util: Use correct address family in set_dscp(), instead of guessing.

The set_dscp() function, until now, tried to set the DSCP as IPv4 and as
IPv6. This worked OK on Linux, where an ENOPROTOOPT error made it really
clear which one was wrong, but FreeBSD uses EINVAL instead, which has
multiple meanings and which it therefore seems somewhat risky to ignore.
Instead, this commit just tries to set the correct address family's DSCP
option.

Tested by Alex Wang on FreeBSD 9.3.

Reported-by: Atanu Ghosh <atanu@acm.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Co-authored-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Tested-by: Alex Wang <alexw@nicira.com>