possibly non-open devices, and we don't want to restrict dumping
to swap devices anwyay. It is especially invalid to call d_ioctl()
in non-process context for panics. d_psize() can be called on
non-open devices, at least on non-SLICED ones that support d_dump(),
and setdumpdev() has depended on this for a long time although it
is probably wrong, but even d_psize() can't be called in non-process
context - that's why dumpsys() depends on previously computed values
although these values may be stale. The historical restriction to
devices with dkpart(dev) == SWAP_PART should go away.
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