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>

lib/netlink-socket.c

@@ -128,8 +128,12 @@ nl_sock_create(int protocol, struct nl_sock **sockp)
     rcvbuf = 1024 * 1024;
     if (setsockopt(sock->fd, SOL_SOCKET, SO_RCVBUFFORCE,
                    &rcvbuf, sizeof rcvbuf)) {
-        VLOG_WARN_RL(&rl, "setting %d-byte socket receive buffer failed (%s)",
-                     rcvbuf, strerror(errno));
+        /* Only root can use SO_RCVBUFFORCE.  Everyone else gets EPERM.
+         * Warn only if the failure is therefore unexpected. */
+        if (errno != EPERM || !getuid()) {
+            VLOG_WARN_RL(&rl, "setting %d-byte socket receive buffer failed "
+                         "(%s)", rcvbuf, strerror(errno));
+        }
     }
 
     retval = get_socket_rcvbuf(sock->fd);