btrfs: support subpage in btrfs_clone_extent_buffer

For btrfs_clone_extent_buffer(), it's mostly the same code of
__alloc_dummy_extent_buffer(), except it has extra page copy.

So to make it subpage compatible, we only need to:

- Call set_extent_buffer_uptodate() instead of SetPageUptodate()
  This will set correct uptodate bit for subpage and regular sector size
  cases.

Since we're calling set_extent_buffer_uptodate() which will also set
EXTENT_BUFFER_UPTODATE bit, we don't need to manually set that bit
either.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>