selinux: use u32 as bit position type in ebitmap code

The extensible bitmap supports bit positions up to U32_MAX due to the
type of the member highbit being u32.  Use u32 consistently as the type
for bit positions to announce to callers what range of values is
supported.

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