perf tests stat+csv_output: Fix shellcheck warnings

perf tests stat+csv_output: Fix shellcheck warnings

Running the shellcheck on stat+csv_output resulted in the following
warning.

Result with shellcheck  without patch:
=====
$ shellcheck -S warning stat+csv_output.sh

In stat+csv_output.sh line 23:
         [ $(uname -m) = "s390x" ] && exp='^[6-7]$'
           ^---------^ SC2046: Quote this to prevent word splitting.
In stat+csv_output.sh line 51:
[ $(id -u) != 0 ] && [ $(cat /proc/sys/kernel/perf_event_paranoid) -gt $1 ]
  ^------^ SC2046: Quote this to prevent word splitting.
                       ^-- SC2046: Quote this to prevent word splitting.
=====

Fixed the warning SC2046 by adding quotes to prevent word splitting.

Result with shellcheck with patch:
=====
$ shellcheck -S warning tests/shell/stat+csv_output.sh
$ ./perf test "stat CSV output linter"
 96: perf stat CSV output linter                                     : Ok
=====

Signed-off-by: Korrapati Likhitha <likhitha@linux.ibm.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230613164145.50488-6-atrajeev@linux.vnet.ibm.com
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>