f2fs: quota: decrease the lock granularity of statfs_project

According to fs/quota/dquot.c, `dq_data_lock' protects mem_dqinfo
structures and modifications of dquot pointers in the inode, and
`dquot->dq_dqb_lock' protects data from dq_dqb.

We should use dquot->dq_dqb_lock in statfs_project instead of
dq_dat_lock.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>