Get rid of dev in rdev and use bdev exclusively.

There is an awkwardness here in that userspace sometimes
passed down a dev_t (e.g. hot_add_disk) and sometime
a major and a minor (e.g. add_new_disk).  Should we convert
both to kdev_t as the uniform standard....
That is what was being done but it seemed very clumsy and
things were gets converted back and forth a lot.

As bdget used a dev_t, I felt safe in staying with dev_t once I
had one rather than converting to kdev_t and back.