socket-util: Remove DSCP_INVALID.

socket-util: Remove DSCP_INVALID.

The DSCP_INVALID flag allowed callers to prevent socket-util from
modify the DSCP bits of newly created sockets.  However, the two
really important callers (implementations of the controller and
manager tables) never used it.  Furthermore, the other callers
would be fine always setting the DSCP bits to zero.  This patch
removes the DSCP_INVALID option in an effort to simplify the code.

Signed-off-by: Ethan Jackson <ethan@nicira.com>