security: smack: add kmem_cache for smack_rule allocations

security: smack: add kmem_cache for smack_rule allocations

On ARM, sizeof(struct smack_rule)==20. Allocation by kmalloc() uses a
32-byte-long chunk to allocate 20 bytes. Just ask ksize().  It means that 40%
of memory is simply wasted for padding bytes.

The problem is fixed in this patch by using kmem_cache. The cache allocates
struct smack_rule using 24-byte-long chunks according to ksize(). This reduces
amount of used memory by 25%.

Change-Id: I2753cabc78c31b695ac07bf76cc8861232b64b1d
Signed-off-by: jooseong.lee <jooseong.lee@samsung.com>