ufs: fix deadlocks introduced by sb mutex merge

Commit 0244756edc4b ("ufs: sb mutex merge + mutex_destroy") introduces
deadlocks in ufs_new_inode() and ufs_free_inode().
Most callers of that functions acqure the mutex by themselves and
ufs_{new,free}_inode() do that via lock_ufs(),
i.e we have an unavoidable double lock.

The patch proposes to resolve the issue by making sure that
ufs_{new,free}_inode() are not called with the mutex held.

Found by Linux Driver Verification project (linuxtesting.org).

Cc: [e-mail] # 3.16
Signed-off-by: Alexey Khoroshilov <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/ufs/inode.c

···
 	invalidate_inode_buffers(inode);
 	clear_inode(inode);
 
-	if (want_delete) {
-		lock_ufs(inode->i_sb);
-		ufs_free_inode (inode);
-		unlock_ufs(inode->i_sb);
-	}
+	if (want_delete)
+		ufs_free_inode(inode);
 }

fs/ufs/namei.c

···
 	if (l > sb->s_blocksize)
 		goto out_notlocked;
 
-	lock_ufs(dir->i_sb);
 	inode = ufs_new_inode(dir, S_IFLNK | S_IRWXUGO);
 	err = PTR_ERR(inode);
 	if (IS_ERR(inode))
-		goto out;
+		goto out_notlocked;
 
+	lock_ufs(dir->i_sb);
 	if (l > UFS_SB(sb)->s_uspi->s_maxsymlinklen) {
 		/* slow symlink */
 		inode->i_op = &ufs_symlink_inode_operations;
···
 	struct inode * inode;
 	int err;
 
-	lock_ufs(dir->i_sb);
-	inode_inc_link_count(dir);
-
 	inode = ufs_new_inode(dir, S_IFDIR|mode);
-	err = PTR_ERR(inode);
 	if (IS_ERR(inode))
-		goto out_dir;
+		return PTR_ERR(inode);
 
 	inode->i_op = &ufs_dir_inode_operations;
 	inode->i_fop = &ufs_dir_operations;
 	inode->i_mapping->a_ops = &ufs_aops;
 
 	inode_inc_link_count(inode);
+
+	lock_ufs(dir->i_sb);
+	inode_inc_link_count(dir);
 
 	err = ufs_make_empty(inode, dir);
 	if (err)
···
 	inode_dec_link_count(inode);
 	inode_dec_link_count(inode);
 	iput (inode);
-out_dir:
 	inode_dec_link_count(dir);
 	unlock_ufs(dir->i_sb);
 	goto out;