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>