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