swap_info_struct became pretty much superfluous.  As we are minimizing
kdev_t usage anyway, I took the opportunity to remove swap_device
member, and replace the remaining usages with SWP_BLOCKDEV bit flag.

Adding SWP_BLOCKDEV in turn motivated a small cleanup of the
SWP_xxx bit flags and their usage.

Patch has been in light testing for a couple weeks, and
has been glanced at by Al.  "looks sane"