btrfs: subpage: make btrfs_submit_compressed_write() compatible

There is a WARN_ON() checking if @start is aligned to PAGE_SIZE, not
sectorsize, which will cause false alert for subpage.  Fix it to check
against sectorsize.

Furthermore:

- Use ASSERT() to do the check
  So that in the future we may skip the check for production build

- Also check alignment for @len

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