btrfs: Make btrfs_(set|clear)_header_flag return void

From the introduction of btrfs_(set|clear)_header_flag, there is no
usage of its return value.  So just make it return void.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>