quota: Use inode->i_blkbits to get block bits

Andrew has suggested to use inode->i_blkbits to get the block bits info,
rather than use super block's blockbits. That should be faster and emit
less code.

Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: Jan Kara <jack@suse.cz>