libsmack: shrink few rule and label struct fields

libsmack: shrink few rule and label struct fields

Use smaller struct fields to optimize memory usage and speed.
On a 32-bit machine this saves 4 bytes per rule and 4 bytes per label.
Limit label length to 8 bits. It's max value is already limited to 255.
Limit label id to 16 bits. While policy with more than 2^16 labels is
theoretically possible, it would be handled very badly by kernel.

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>