btrfs: support subpage for extent buffer page release

In btrfs_release_extent_buffer_pages(), we need to add extra handling
for subpage.

Introduce a helper, detach_extent_buffer_page(), to do different
handling for regular and subpage cases.

For subpage case, handle detaching page private.

For unmapped (dummy or cloned) ebs, we can detach the page private
immediately as the page can only be attached to one unmapped eb.

For mapped ebs, we have to ensure there are no eb in the page range
before we delete it, as page->private is shared between all ebs in the
same page.

But there is a subpage specific race, where we can race with extent
buffer allocation, and clear the page private while new eb is still
being utilized, like this:

  Extent buffer A is the new extent buffer which will be allocated,
  while extent buffer B is the last existing extent buffer of the page.

  		T1 (eb A) 	 |		T2 (eb B)
  -------------------------------+------------------------------
  alloc_extent_buffer()		 | btrfs_release_extent_buffer_pages()
  |- p = find_or_create_page()   | |
  |- attach_extent_buffer_page() | |
  |				 | |- detach_extent_buffer_page()
  |				 |    |- if (!page_range_has_eb())
  |				 |    |  No new eb in the page range yet
  |				 |    |  As new eb A hasn't yet been
  |				 |    |  inserted into radix tree.
  |				 |    |- btrfs_detach_subpage()
  |				 |       |- detach_page_private();
  |- radix_tree_insert()	 |

  Then we have a metadata eb whose page has no private bit.

To avoid such race, we introduce a subpage metadata-specific member,
btrfs_subpage::eb_refs.

In alloc_extent_buffer() we increase eb_refs in the critical section of
private_lock.  Then page_range_has_eb() will return true for
detach_extent_buffer_page(), and will not detach page private.

The section is marked by:

- btrfs_page_inc_eb_refs()
- btrfs_page_dec_eb_refs()

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