btrfs: scrub: drop a few function declarations

Drop function declarations at the beginning of the file scrub.c. These
functions are defined before they are used in the same file and don't
need forward declaration.

No functional changes.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>