[BUG]
With current btrfs subpage rw support, the following script can lead to
fs hang:

  $ mkfs.btrfs -f -s 4k $dev
  $ mount $dev -o nospace_cache $mnt
  $ fsstress -w -n 100 -p 1 -s 1608140256 -v -d $mnt

The fs will hang at btrfs_start_ordered_extent().

[CAUSE]
In above test case, btrfs_invalidate() will be called with the following
parameters:

  offset = 0 length = 53248 page dirty = 1 subpage dirty bitmap = 0x2000

Since @offset is 0, btrfs_invalidate() will try to invalidate the full
page, and finally call clear_page_extent_mapped() which will detach
subpage structure from the page.

And since the page no longer has subpage structure, the subpage dirty
bitmap will be cleared, preventing the dirty range from being written
back, thus no way to wake up the ordered extent.

[FIX]
Just follow other filesystems, only to invalidate the page if the range
covers the full page.

There are cases like truncate_setsize() which can call
btrfs_invalidatepage() with offset == 0 and length != 0 for the last
page of an inode.

Although the old code will still try to invalidate the full page, we are
still safe to just wait for ordered extent to finish.
So it shouldn't cause extra problems.

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>