selinux: make left shifts well defined

The loops upper bound represent the number of permissions used (for the
current class or in general).  The limit for this is 32, thus we might
left shift of one less, 31.  Shifting a base of 1 results in undefined
behavior; use (u32)1 as base.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>