perf tests: Fix shellcheck warning in record_sideband.sh

perf tests: Fix shellcheck warning in record_sideband.sh

Running shellcheck on record_sideband.sh throws below
warning:

	In tests/shell/record_sideband.sh line 25:
	  if ! perf record -o ${perfdata} -BN --no-bpf-event -C $1 true 2>&1 >/dev/null
	    ^--^ SC2069: To redirect stdout+stderr, 2>&1 must be last (or use '{ cmd > file; } 2>&1' to clarify).

This shows shellcheck warning SC2069 where the redirection
order needs to be fixed. Use "cmd > /dev/null 2>&1" to fix
the redirection of perf record output

Fixes: 23b97c7ee963 ("perf test: Add test case for record sideband events")
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Reviewed-by: Kajol Jain <kjain@linux.ibm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: disgoel@linux.vnet.ibm.com
Link: https://lore.kernel.org/r/20231013073021.99794-3-atrajeev@linux.vnet.ibm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>