swim: add helper for disk cleanup

Disk cleanup can be shared between exit and bringup. Use a
helper to do the work required. The only functional change at
this point is we're being overly paraoid on exit to check for
a null disk as well now, and this should be safe.

We'll later expand on this, this change just makes subsequent
changes easier to read.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20210927220302.1073499-9-mcgrof@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>