Fix rdev field in cpio format for device nodes

Fix rdev field in cpio format for device nodes

Currently, the rdev field is set from archive_entry_dev, which is
the device number of the filesystem containing the device node, not
the device itself. It should instead use archive_entry_rdev.

Here's an example demonstrating the problem:

$ ls -l /dev/null
crw-rw-rw-    1 root     root        1,    3 May 26 02:03 /dev/null
$ bsdtar --format=cpio -cf null.cpio /dev/null
bsdtar: Removing leading '/' from member names
$ bsdtar -tvf null.cpio
crw-rw-rw-  1 0      0         0,6 May 26 02:03 dev/null