btrfs: wait for writeback if sector size is smaller than page size

btrfs: wait for writeback if sector size is smaller than page size

[PROBLEM]
If sector perfect compression is enabled for sector size < page size
case, the following case can lead dirty ranges not being written back:

     0     32K     64K     96K     128K
     |     |///////||//////|     |/|
                                 124K

In above example, the page size is 64K, and we need to write back above
two pages.

- Submit for page 0 (main thread)
  We found delalloc range [32K, 96K), which can be compressed.
  So we queue an async range for [32K, 96K).
  This means, the page unlock/clearing dirty/setting writeback will
  all happen in a workqueue context.

- The compression is done, and compressed range is submitted (workqueue)
  Since the compression is done in asynchronously, the compression can
  be done before the main thread to submit for page 64K.

  Now the whole range [32K, 96K), involving two pages, will be marked
  writeback.

- Submit for page 64K (main thread)
  extent_write_cache_pages() got its wbc->sync_mode is WB_SYNC_NONE,
  so it skips the writeback wait.

  And unlock the page and exit. This means the dirty range [124K, 128K)
  will never be submitted, until next writeback happens for page 64K.

This will never happen for previous kernels because:

- For sector size == page size case
  Since one page is one sector, if a page is marked writeback it will
  not have dirty flags.
  So this corner case will never hit.

- For sector size < page size case
  We never do subpage compression, a range can only be submitted for
  compression if the range is fully page aligned.
  This change makes the subpage behavior mostly the same as non-subpage
  cases.

[ENHANCEMENT]
Instead of relying WB_SYNC_NONE check only, if it's a subpage case, then
always wait for writeback flags.

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