btrfs: subpage: prepare for large data folios

btrfs: subpage: prepare for large data folios

The subpage handling code has two locations not supporting large folios:

- btrfs_attach_subpage()
  Which is doing a metadata specific ASSERT() check.

  But for the future large data folios support, that check is too
  generic.  Since it's metadata specific, only check the ASSERT() for
  metadata.

- btrfs_subpage_assert()
  Just remove the "ASSERT(folio_order(folio) == 0)" check.

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