gfs2: use i_lock spin_lock for inode qadata

Before this patch, functions gfs2_qa_get and _put used the i_rw_mutex to
prevent simultaneous access to its i_qadata. But i_rw_mutex is now used
for many other things, including iomap_begin and end, which causes a
conflict according to lockdep. We cannot just remove the lock since
simultaneous opens (gfs2_open -> gfs2_open_common -> gfs2_qa_get) can
then stomp on each others values for i_qadata.

This patch solves the conflict by using the i_lock spin_lock in the inode
to prevent simultaneous access.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/quota.c

@@ -528,34 +528,42 @@ static void qdsb_put(struct gfs2_quota_data *qd)
  */
 int gfs2_qa_get(struct gfs2_inode *ip)
 {
-	int error = 0;
 	struct gfs2_sbd *sdp = GFS2_SB(&ip->i_inode);
+	struct inode *inode = &ip->i_inode;
 
 	if (sdp->sd_args.ar_quota == GFS2_QUOTA_OFF)
 		return 0;
 
-	down_write(&ip->i_rw_mutex);
+	spin_lock(&inode->i_lock);
 	if (ip->i_qadata == NULL) {
-		ip->i_qadata = kmem_cache_zalloc(gfs2_qadata_cachep, GFP_NOFS);
-		if (!ip->i_qadata) {
-			error = -ENOMEM;
-			goto out;
-		}
+		struct gfs2_qadata *tmp;
+
+		spin_unlock(&inode->i_lock);
+		tmp = kmem_cache_zalloc(gfs2_qadata_cachep, GFP_NOFS);
+		if (!tmp)
+			return -ENOMEM;
+
+		spin_lock(&inode->i_lock);
+		if (ip->i_qadata == NULL)
+			ip->i_qadata = tmp;
+		else
+			kmem_cache_free(gfs2_qadata_cachep, tmp);
 	}
 	ip->i_qadata->qa_ref++;
-out:
-	up_write(&ip->i_rw_mutex);
-	return error;
+	spin_unlock(&inode->i_lock);
+	return 0;
 }
 
 void gfs2_qa_put(struct gfs2_inode *ip)
 {
-	down_write(&ip->i_rw_mutex);
+	struct inode *inode = &ip->i_inode;
+
+	spin_lock(&inode->i_lock);
 	if (ip->i_qadata && --ip->i_qadata->qa_ref == 0) {
 		kmem_cache_free(gfs2_qadata_cachep, ip->i_qadata);
 		ip->i_qadata = NULL;
 	}
-	up_write(&ip->i_rw_mutex);
+	spin_unlock(&inode->i_lock);
 }
 
 int gfs2_quota_hold(struct gfs2_inode *ip, kuid_t uid, kgid_t gid)