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.