perf test stat_bpf_counters_cgrp: Fix shellcheck issue about logical operators

Running shellcheck on lock_contention.sh generates below warning:

In stat_bpf_counters_cgrp.sh line 28:
	if [ -d /sys/fs/cgroup/system.slice -a -d /sys/fs/cgroup/user.slice ]; then
                                            ^-- SC2166 (warning): Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.

In stat_bpf_counters_cgrp.sh line 34:
	local self_cgrp=$(grep perf_event /proc/self/cgroup | cut -d: -f3)
        ^-------------^ SC3043 (warning): In POSIX sh, 'local' is undefined.
              ^-------^ SC2155 (warning): Declare and assign separately to avoid masking return values.
                        ^-- SC2046 (warning): Quote this to prevent word splitting.

In stat_bpf_counters_cgrp.sh line 51:
	local output
        ^----------^ SC3043 (warning): In POSIX sh, 'local' is undefined.

In stat_bpf_counters_cgrp.sh line 65:
	local output
        ^----------^ SC3043 (warning): In POSIX sh, 'local' is undefined.

Fixed above warnings by:
- Changing the expression [p -a q] to [p] && [q].
- Fixing shellcheck warnings for local usage, by prefixing
  function name to the variable.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230709182800.53002-6-atrajeev@linux.vnet.ibm.com


Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>