libkmod-hash: always align n_buckets to power of 2

libkmod-hash: always align n_buckets to power of 2

By aligning n_buckets to power of 2 we can turn the "bucket = hashval %
n_buckets" into a less expensive bucket = hashval & (n_buckets - 1).
This removes the DIV instruction as shown below.

Before:
	xor    %edx,%edx
	divl   0x8(%rbx)
	mov    %edx,%eax
	add    $0x1,%rax
	shl    $0x4,%rax
	add    %rbx,%rax

After:
	lea    -0x1(%rdi),%edx
	and    %edx,%eax
	add    $0x1,%rax
	shl    $0x4,%rax
	add    %rbx,%rax

With a microbenchmark, measuring the time to locate the bucket (i.e.
time_to_calculate_hashval + time_to_calculate_bucket_position) we have
the results below (time in clock cycles):

	keylen      before   after
	2-10          79.0    61.9 (-21.65%)
	11-17         81.0    64.4 (-20.48%)
	18-25         90.0    73.2 (-18.69%)
	26-32        104.7    87.0 (-16.82%)
	33-40        108.4    89.6 (-17.37%)
	41-48        111.2    91.9 (-17.38%)
	49-55        120.1   102.1 (-15.04%)
	56-63        134.4   115.7 (-13.91%)

As expected the gain is constant, regardless of the key length.
The time to clculate the hashval varies with the key length, which
explains the bigger gains for short keys.