btrfs-progs: fix improper output msg for btrfs-fi-usage

btrfs-progs: fix improper output msg for btrfs-fi-usage

Even if run as root:
	# su
	# btrfs file usage <path> <== path exits outside the mnt point
We get the output:
	WARNING: ..., run as root
	WARNING: ..., run as root
	ERROR:...

It is because in load_chunk_info, the errno of ioctl is not judged
but rather the ret value of ioctl is judged. And the ret value of
ioctl is -1 which happens to match -EPERM exactly.
So the outer warning is printed.

Just judge the errno of ioctl and prevent the ret value of load_chunk_info
to be -1 in other error conditions.

For load_device_info, the problem and fix is the same.
After the fix, the 'run as root' WARNINGs will not show up in this condition.

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