btrfs: cow_file_range() num_bytes and disk_num_bytes are same

This patch deletes local variable disk_num_bytes as its value
is same as num_bytes in the function cow_file_range().

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>