expose device number for character and block devices

[joachimmetz]

Device numbers are presumably stored in data fork, based on: https://web.archive.org/web/20171031110501/http://oss.sgi.com/projects/xfs/papers/xfs_filesystem_structure.pdf#page=25

For character and block devices (S_IFCHR and S_IFBLK), cast the value from XFS_DFORK_DPTR to xfs_dev_t*

some sources define xfs_dev_t as a 32-bit unsigned integer