btrfs-progs: fix error returns in get_df()

btrfs-progs: fix error returns in get_df()

get_df returns -ERRNO, or maybe (+)errno, or even 0 in
the case where we inexplicably got 0 total_spaces from
the BTRFS_IOC_SPACE_INFO.

Consistently return a negative error number, and return
-ENOENT rather than 0 for total_spaces == 0, so that the
caller will know that **sargs_ret hasn't been set up.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>