btrfs-progs: fix return value problem for btrfs sub show

btrfs-progs: fix return value problem for btrfs sub show

If you exec:
	# btrfs sub show <dir>	<== non-subvolume dir
The cmd print error messages as expected, but returns 0.
By convetion, it should return non-zero and we should explicitly
set it before it goto out.

With other pieces adopted:
1) removed a unnecessary return value set -EINVAL
2) fixed another code branch which may return 0 upon error.
3) with 2) applied, the ret = 0 follows can be removed

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