btrfs-progs: Fix btrfs fi show by uuid and label

btrfs-progs: Fix btrfs fi show by uuid and label

Commit 8be2fff (btrfs-progs: apply realpath for btrfs fi
show when mount point is given) changed the behavior of
btrfs fi show to return an error if the call to realpath()
failed.  This broke the ability to specify a filesystem by
uuid or label.

So let's not consider a failed call to realpath() as an
error.  If the user really specified a bad device, just
return nothing like we did before.

Signed-off-by: Justin Maggard <jmaggard10@gmail.com>
Reviewed-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>