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>