kselftest/cgroup: Fix gathering number of CPUs

test_cpuset_prs.sh is failing with the following error:

	test_cpuset_prs.sh: line 29: [[: 8
	57%: syntax error in expression (error token is "57%")

This is happening because `lscpu | grep "^CPU(s)"` returns two lines in
some systems (such as Debian unstable):

	# lscpu | grep "^CPU(s)"
	CPU(s):                          8
	CPU(s) scaling MHz:              55%

This is a simple fix that discard the second line.
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/cgroup/test_cpuset_prs.sh

@@ -25,7 +25,7 @@ WAIT_INOTIFY=$(cd $(dirname $0); pwd)/wait_inotify
 CGROUP2=$(mount -t cgroup2 | head -1 | awk -e '{print $3}')
 [[ -n "$CGROUP2" ]] || skip_test "Cgroup v2 mount point not found!"
 
-CPUS=$(lscpu | grep "^CPU(s)" | sed -e "s/.*:[[:space:]]*//")
+CPUS=$(lscpu | grep "^CPU(s):" | sed -e "s/.*:[[:space:]]*//")
 [[ $CPUS -lt 8 ]] && skip_test "Test needs at least 8 cpus available!"
 
 # Set verbose flag and delay factor