quota: Get rid of nested I_MUTEX_QUOTA locking subclass

So far i_mutex was ranking above dqonoff_mutex and i_mutex on quota files
was special and ranking below dqonoff_mutex (and several other locks).
However there's no real need for i_mutex on quota files to be special.
IO on quota files is serialized by dqio_mutex anyway so we don't need to
take i_mutex when writing to quota files. Other places where we take i_mutex
on quota file can accomodate standard i_mutex lock ranking, we only need
to change the lock ranking to be dqonoff_mutex > i_mutex which is a matter
of changing documentation because there's no place which would enforce
ordering in the other direction.

Signed-off-by: Jan Kara <[e-mail]>

fs/quota/dquot.c

···
  * spinlock to internal buffers before writing.
  *
  * Lock ordering (including related VFS locks) is the following:
- *   i_mutex > dqonoff_sem > journal_lock > dqptr_sem > dquot->dq_lock >
+ *   dqonoff_mutex > i_mutex > journal_lock > dqptr_sem > dquot->dq_lock >
  *   dqio_mutex
+ * dqonoff_mutex > i_mutex comes from dquot_quota_sync, dquot_enable, etc.
  * The lock ordering of dqptr_sem imposed by quota code is only dqonoff_sem >
  * dqptr_sem. But filesystem has to count with the fact that functions such as
  * dquot_alloc_space() acquire dqptr_sem and they usually have to be called
  * from inside a transaction to keep filesystem consistency after a crash. Also
  * filesystems usually want to do some IO on dquot from ->mark_dirty which is
  * called with dqptr_sem held.
- * i_mutex on quota files is special (it's below dqio_mutex)
  */
 
 static __cacheline_aligned_in_smp DEFINE_SPINLOCK(dq_list_lock);
···
 			continue;
 		if (!sb_has_quota_active(sb, cnt))
 			continue;
-		mutex_lock_nested(&dqopt->files[cnt]->i_mutex,
-				  I_MUTEX_QUOTA);
+		mutex_lock(&dqopt->files[cnt]->i_mutex);
 		truncate_inode_pages(&dqopt->files[cnt]->i_data, 0);
 		mutex_unlock(&dqopt->files[cnt]->i_mutex);
 	}
···
 			/* If quota was reenabled in the meantime, we have
 			 * nothing to do */
 			if (!sb_has_quota_loaded(sb, cnt)) {
-				mutex_lock_nested(&toputinode[cnt]->i_mutex,
-						  I_MUTEX_QUOTA);
+				mutex_lock(&toputinode[cnt]->i_mutex);
 				toputinode[cnt]->i_flags &= ~(S_IMMUTABLE |
 				  S_NOATIME | S_NOQUOTA);
 				truncate_inode_pages(&toputinode[cnt]->i_data,
···
 		/* We don't want quota and atime on quota files (deadlocks
 		 * possible) Also nobody should write to the file - we use
 		 * special IO operations which ignore the immutable bit. */
-		mutex_lock_nested(&inode->i_mutex, I_MUTEX_QUOTA);
+		mutex_lock(&inode->i_mutex);
 		oldflags = inode->i_flags & (S_NOATIME | S_IMMUTABLE |
 					     S_NOQUOTA);
 		inode->i_flags |= S_NOQUOTA | S_NOATIME | S_IMMUTABLE;
···
 	iput(inode);
 out_lock:
 	if (oldflags != -1) {
-		mutex_lock_nested(&inode->i_mutex, I_MUTEX_QUOTA);
+		mutex_lock(&inode->i_mutex);
 		/* Set the flags back (in the case of accidental quotaon()
 		 * on a wrong file we don't want to mess up the flags) */
 		inode->i_flags &= ~(S_NOATIME | S_NOQUOTA | S_IMMUTABLE);