bcachefs: Add a progress indicator to bch2_dev_data_drop()

bcachefs: Add a progress indicator to bch2_dev_data_drop()

This code needs quite a bit of work: we don't want to be walking all
metadata in the filesystem, we should just be walking backpointers, and
it should be switched to a data ioctl that can report progress via a
file descriptor, not the system console.

But that'll take more work - before we can safely walk only backpointers
we need to change device add to not reuse device indexes, since with
that change accounting being wrong introduces the possibility of
removing a device that still has pointers.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>