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>