ext2: Convert to using invalidate_lock

Ext2 has its private dax_sem used for synchronizing page faults and
truncation. Use mapping->invalidate_lock instead as it is meant for this
purpose.

CC: <linux-ext4@vger.kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>