selinux: simplify avtab slot calculation

Instead of dividing by 8 and then performing log2 by hand, use a more
readable calculation.

The behavior of rounddown_pow_of_two() for an input of 0 is undefined,
so handle that case and small values manually to achieve the same
results.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: cgzones

security/selinux/ss/avtab.c

@@ -298,13 +298,7 @@ int avtab_alloc(struct avtab *h, u32 nrules)
 	u32 nslot = 0;
 
 	if (nrules != 0) {
-		u32 shift = 1;
-		u32 work = nrules >> 3;
-		while (work) {
-			work >>= 1;
-			shift++;
-		}
-		nslot = 1 << shift;
+		nslot = nrules > 3 ? rounddown_pow_of_two(nrules / 2) : 2;
 		if (nslot > MAX_AVTAB_HASH_BUCKETS)
 			nslot = MAX_AVTAB_HASH_BUCKETS;