ext4: fix data corruption regression by reverting commit 6de9843d

Revert commit 6de9843d, since it
caused a data corruption regression with BitTorrent downloads.  Thanks
to Damien for discovering and bisecting to find the problem commit.

https://bugzilla.kernel.org/show_bug.cgi?id=32972Reported-by: NDamien Grassart <damien@grassart.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -2502,6 +2502,7 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
 		 * for partial write.
 		 */
 		set_buffer_new(bh);
+		set_buffer_mapped(bh);
 	}
 	return 0;
 }