Drop the single-use variable bytenr_orig and instead use btrfs_sb_offset()
in the function argument passing.

Fix a stale comment about not automatically fixing a bad primary
superblock from the backup mirror copies. Also, move the comment closer
to where the primary superblock read occurs.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>