btrfs: Use correct format specifier

Component mirror_num of struct btrfsic_block is defined
as unsigned int. Use %u as format specifier.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>