btrfs: remove duplicate const specifier

commit fb75d857a31d600cc0c37b8c7d914014f7fa3f9a upstream.

duplicate const is redundant so remove it

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>