btrfs: subpage: make alloc_extent_buffer() handle previously uptodate range efficiently

Currently alloc_extent_buffer() would make the extent buffer uptodate if
the corresponding pages are also uptodate.

But this check is only checking PageUptodate, which is fine for regular
cases, but not for subpage cases, as we can have multiple extent buffers
in the same page.

So here we go btrfs_page_test_uptodate() instead.

The old code doesn't cause any problem, but is not efficient, as it
would cause extra metadata read even if the range is already uptodate.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>