btrfs-progs: fix a printf format string fatal warning
    
    At least in Debian, default build flags include -Werror=format-security,
    for good reasons in most cases.  Here, the string comes from strftime --
    and though I don't suspect any locale would be crazy enough to have %X
    include a '%' char, the compiler has no way to know that.
    
    Signed-off-by: Adam Borowski <kilobyte@angband.pl>
    Signed-off-by: David Sterba <dsterba@suse.com>