quota: code cleanup for hash bits calculation

Code cleanup for hash bits calculation by
calling ilog2().

Link: https://lore.kernel.org/r/20190923135223.27674-1-cgxu519@zoho.com.cn


Signed-off-by: Chengguang Xu <cgxu519@zoho.com.cn>
Signed-off-by: Jan Kara <jack@suse.cz>