sysctl: return error on permission denied

When trying to write to a readonly sysctl or a directory by mistake,
sysctl still returns success:

	# sysctl kernel.version=foo
	sysctl: setting key "kernel.version": Operation not permitted
	# echo $?
	0
	# sysctl kernel=foo
	sysctl: setting key "kernel": Operation not permitted
	# echo $?
	0

Let sysctl return error in both the cases above.

Signed-off-by: Matteo Croce <teknoraver@meta.com>