btrfs: prevent extent_clear_unlock_delalloc() to unlock page not locked by __process_pages_contig()

In cow_file_range(), after we have succeeded creating an inline extent,
we unlock the page with extent_clear_unlock_delalloc() by passing
locked_page == NULL.

For sectorsize == PAGE_SIZE case, this is just making the page lock and
unlock harder to grab.

But for incoming subpage case, it can be a big problem.

For incoming subpage case, page locking have two entry points:

- __process_pages_contig()
  In that case, we know exactly the range we want to lock (which only
  requires sector alignment).
  To handle the subpage requirement, we introduce btrfs_subpage::writers
  to page::private, and will update it in __process_pages_contig().

- Other directly lock/unlock_page() call sites
  Those won't touch btrfs_subpage::writers at all.

This means, page locked by __process_pages_contig() can only be unlocked
by __process_pages_contig().
Thankfully we already have the existing infrastructure in the form of
@locked_page in various call sites.

Unfortunately, extent_clear_unlock_delalloc() in cow_file_range() after
creating an inline extent is the exception.
It intentionally call extent_clear_unlock_delalloc() with locked_page ==
NULL, to also unlock current page (and clear its dirty/writeback bits).

To co-operate with incoming subpage modifications, and make the page
lock/unlock pair easier to understand, this patch will still call
extent_clear_unlock_delalloc() with locked_page, and only unlock the
page in __extent_writepage().

Tested-by: Ritesh Harjani <riteshh@linux.ibm.com> # [ppc64]
Tested-by: Anand Jain <anand.jain@oracle.com> # [aarch64]
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>