btrfs: clarify btrfs_lookup_bio_sums documentation

btrfs: clarify btrfs_lookup_bio_sums documentation

Fix a couple of issues in the btrfs_lookup_bio_sums documentation:

* The bio doesn't need to be a btrfs_io_bio if dst was provided. Move
  the declaration in the code to make that clear, too.
* dst must be large enough to hold nblocks * csum_size, not just
  csum_size.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>