ext3: Don't use MAXQUOTAS value

MAXQUOTAS value defines maximum number of quota types VFS supports. This
isn't necessarily the number of types ext3 supports and with addition of
project quotas these two numbers stop matching. So make ext3 use its
private definition.

CC: linux-ext4@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>