selinux: avoid implicit conversions in avtab code

Return u32 from avtab_hash() instead of int, since the hashing is done
on u32 and the result is used as an index on the hash array.

Use the type of the limit in for loops.

Avoid signed to unsigned conversion of multiplication result in
avtab_hash_eval() and perform multiplication in destination type.

Use unsigned loop iterator for index operations, to avoid sign
extension.

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