btrfs-progs: clean fsck noise for free inode generation 0

btrfs-progs: clean fsck noise for free inode generation 0

When we write a btrfs to full and then we have no space left for
free space cache.
The btrfs check will output msg as follows which is noise indeed:
	# free space inode generation (0) did not match
	  free space cache generation (XXX)

When the free space cache is not written out normally,
the free inode generation will be 0.
In this condition, no noise should be outputed.
Also, check 0-sized inode eariler together with 0-generationed inode.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>