btrfs: subpage: only allow compression if the range is fully page aligned

For compressed write, we use a mechanism called async COW, which unlike
regular run_delalloc_cow() or cow_file_range() will also unlock the
first page.

This mechanism allows us to continue handling next ranges, without
waiting for the time consuming compression.

But this has a problem for subpage case, as we could have the following
delalloc range for a page:

0		32K		64K
|	|///////|	|///////|
		\- A		\- B

In the above case, if we pass both ranges to cow_file_range_async(),
both range A and range B will try to unlock the full page [0, 64K).

And which one finishes later than the other one will try to do other
page operations like end_page_writeback() on a unlocked page, triggering
VM layer BUG_ON().

To make subpage compression work at least partially, here we add another
restriction for it, only allow compression if the delalloc range is
fully page aligned.

By that, async extent is always ensured to unlock the first page
exclusively, just like it used to be for regular sectorsize.

In theory, we only need to make sure the delalloc range fully covers its
first page, but the tail page will be locked anyway, blocking later
writeback until the compression finishes.

Thus here we choose to make sure the range is fully page aligned before
doing the compression.

In the future, we could optimize the situation by properly increasing
subpage::writers number for the locked page, but that also means we need
to change how we run delalloc range of page.
(Instead of running each delalloc range we hit, we need to find and lock
all delalloc ranges covering the page, then run each of them).

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