selftests: mptcp: diag: fix bash warnings on older kernels

Since the 'Fixes' commit mentioned below, the command that is executed
in __chk_nr() helper can return nothing if the feature is not supported.
This is the case when the MPTCP CURRESTAB counter is not supported.

To avoid this warning ...

  ./diag.sh: line 65: [: !=: unary operator expected

... we just need to surround '$nr' with double quotes, to support an
empty string when the feature is not supported.

Fixes: 81ab7728 ("selftests: mptcp: diag: check CURRESTAB counters")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/mptcp/diag.sh

@@ -62,8 +62,8 @@ __chk_nr()
 	nr=$(eval $command)
 
 	printf "%-50s" "$msg"
-	if [ $nr != $expected ]; then
-		if [ $nr = "$skip" ] && ! mptcp_lib_expect_all_features; then
+	if [ "$nr" != "$expected" ]; then
+		if [ "$nr" = "$skip" ] && ! mptcp_lib_expect_all_features; then
 			echo "[ skip ] Feature probably not supported"
 			mptcp_lib_result_skip "${msg}"
 		else