src/t_dir_type.c: Use strtoul() instead of atoll()

src/t_dir_type.c: Use strtoul() instead of atoll()

An overlayfs filesystem instance with one lowerdir filesystem and
with "xino" mount option enabled can have the layer index encoded in
the 63rd bit of the inode number. A signed 64 bit integer won't
suffice to store this inode number. Hence this commit uses strtoul()
to convert the inode number in string form to unsigned integer form.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>