ext4: reserve xattr index for Rich ACL support

Jan Kara <jack@suse.cz>

SUSE is carrying out of tree patches for Rich ACL support for ext4 as
they didn't get upstream due to opposition of some VFS maintainers.
Reserve xattr index for Rich ACLs so that it cannot be taken by
anything else which would force users to backup and reset their Rich
ACLs on files.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>