btrfs: scrub: refactor scrub_find_csum()

Function scrub_find_csum() is to locate the csum for bytenr @logical
from sctx->csum_list.

However it lacks a lot of comments to explain things like how the
csum_list is organized and why we need to drop csum range which is
before us.

Refactor the function by:

- Add more comments explaining the behavior
- Add comment explaining why we need to drop the csum range
- Put the csum copy in the main loop
  This is mostly for the incoming patches to make scrub_find_csum() able
  to find multiple checksums.

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