selinux: consistently use u32 as sequence number type in the status code

Align the type with the one used in selinux_notify_policy_change() and
the sequence member of struct selinux_kernel_status.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: subject line tweaks]
Signed-off-by: Paul Moore <paul@paul-moore.com>