Btrfs-progs: don't output baffling message when checking a fresh fs

Btrfs-progs: don't output baffling message when checking a fresh fs

As we know,  a new fs doesn't have space cache, so we set the cache generation
of the super block to be -1ULL, it is not equal to the fs generation. But the
check program didn't consider this case, and output the following message

  cache and super generation don't match, space cache will be invalidated

directly, it would be baffling the users. So we should avoid outputing such
message. This patch fixes this problem.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>