[BUG]
There is a very rare ASSERT() triggering during full fstests run for
subpage rw support.

No other reproducer so far.

The ASSERT() gets triggered for metadata read in
btrfs_page_set_uptodate() inside end_page_read().

[CAUSE]
There is still a small race window for metadata only, the race could
happen like this:

                T1                  |              T2
------------------------------------+-----------------------------
end_bio_extent_readpage()           |
|- btrfs_validate_metadata_buffer() |
|  |- free_extent_buffer()          |
|     Still have 2 refs             |
|- end_page_read()                  |
   |- if (unlikely(PagePrivate())   |
   |  The page still has Private    |
   |                                | free_extent_buffer()
   |                                | |  Only one ref 1, will be
   |                                | |  released
   |                                | |- detach_extent_buffer_page()
   |                                |    |- btrfs_detach_subpage()
   |- btrfs_set_page_uptodate()     |
      The page no longer has Private|
      >>> ASSERT() triggered <<<    |

This race window is super small, thus pretty hard to hit, even with so
many runs of fstests.

But the race window is still there, we have to go another way to solve
it other than relying on random PagePrivate() check.

Data path is not affected, as it will lock the page before reading,
while unlocking the page after the last read has finished, thus no race
window.

[FIX]
This patch will fix the bug by repurposing btrfs_subpage::readers.

Now btrfs_subpage::readers will be a member shared by both metadata and
data.

For metadata path, we don't do the page unlock as metadata only relies
on extent locking.

At the same time, teach page_range_has_eb() to take
btrfs_subpage::readers into consideration.

So that even if the last eb of a page gets freed, page::private won't be
detached as long as there still are pending end_page_read() calls.

By this we eliminate the race window, this will slight increase the
metadata memory usage, as the page may not be released as frequently as
usual.  But it should not be a big deal.

The code got introduced in ("btrfs: submit read time repair only for
each corrupted sector"), but the fix is in a separate patch to keep the
problem description and the crash is rare so it should not hurt
bisectability.
Signed-off-by: Qu Wegruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>