mountpoint: fix a minor bug with 0:0 devno

mountpoint: fix a minor bug with 0:0 devno

zhi@debian-lizhi:~$ mountpoint /sys
/sys is not a mountpoint
zhi@debian-lizhi:~$

The problem is in mountpoint.c, function dir_to_device(), which uses 0 as an
error return value, but for sysfs 0 is its device number:

zhi@debian-lizhi:~$ cat /proc/self/mountinfo
14 19 0:0 / /sys rw,nosuid,nodev,noexec,relatime - sysfs none rw
15 19 0:3 / /proc rw,nosuid,nodev,noexec,relatime - proc none rw

My change is on dir_to_device, seperating device number from return value.

Signed-off-by: Zhi Li <lizhi1215@gmail.com>