Btrfs: disable xattr operations on subvolume directories

When you snapshot a subvolume containing a subvolume, you get a
placeholder directory where the subvolume would be. These directory
inodes have ->i_ops set to btrfs_dir_ro_inode_operations. Previously,
these i_ops didn't include the xattr operation callbacks. The conversion
to xattr_handlers missed this case, leading to bogus attempts to set
xattrs on these inodes. This manifested itself as failures when running
delayed inodes.

To fix this, clear IOP_XATTR in ->i_opflags on these inodes.

Fixes: 6c6ef9f2 ("xattr: Stop calling {get,set,remove}xattr inode operations")
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Reported-by: NChris Murphy <lists@colorremedies.com>
Tested-by: NChris Murphy <lists@colorremedies.com>
Cc: <stable@vger.kernel.org> # 4.9.x
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/inode.c

@@ -5718,6 +5718,7 @@ static struct inode *new_simple_dir(struct super_block *s,
 
 	inode->i_ino = BTRFS_EMPTY_SUBVOL_DIR_OBJECTID;
 	inode->i_op = &btrfs_dir_ro_inode_operations;
+	inode->i_opflags &= ~IOP_XATTR;
 	inode->i_fop = &simple_dir_operations;
 	inode->i_mode = S_IFDIR | S_IRUGO | S_IWUSR | S_IXUGO;
 	inode->i_mtime = current_time(inode);