btrfs: don't split NOCOW extent_maps in btrfs_extract_ordered_extent

NOCOW writes just overwrite an existing extent map, which thus should
not be split in btrfs_extract_ordered_extent.  The NOCOW case can't
currently happen as btrfs_extract_ordered_extent is only used on zoned
devices that do not support NOCOW writes, but this will change soon.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Boris Burkov <boris@bur.io>
[ hch: split from a larger patch, wrote a commit log ]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -2621,6 +2621,14 @@ int btrfs_extract_ordered_extent(struct btrfs_bio *bbio,
 	ret = btrfs_split_ordered_extent(ordered, len);
 	if (ret)
 		return ret;
+
+	/*
+	 * Don't split the extent_map for NOCOW extents, as we're writing into
+	 * a pre-existing one.
+	 */
+	if (test_bit(BTRFS_ORDERED_NOCOW, &ordered->flags))
+		return 0;
+
 	return split_extent_map(inode, bbio->file_offset, ordered_len, len);
 }