dumpstate: Dump device-mapper info via gsid.

Example spew:

    Device system_ext_a (253:4)
        linear 0 15288 8:14 6197504
    Device userdata (253:6)
        default-key 0 96124888 AES-256-XTS - 8:15 0

Bug: 140204341
Test: dumpstate
Change-Id: I1f099ad6898f458d1839be6c98356480627ea6cb