btrfs: save the bio iter for checksum validation in common code

All callers of btrfs_submit_bio that want to validate checksums
currently have to store a copy of the iter in the btrfs_bio.  Move
the assignment into common code.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>