netlink-socket: Don't bother logging SO_RCVBUFFORCE failure as non-root.

netlink-socket: Don't bother logging SO_RCVBUFFORCE failure as non-root.

Some Open vSwitch utilities can do useful work when they are not run as
root.  Without this commit, these utilities will log a warning on failure
to use the SO_RCVBUFFORCE socket option if they open any Netlink sockets.
This will always happen, it does not report anything unexpected or
fixable as non-root, and sometimes it makes users wonder if something is
wrong, so there is no benefit to logging it.  This commit drops it in that
case.

Signed-off-by: Ben Pfaff <blp@nicira.com>