bcache: Populate writeback_rate_minimum attribute

Forgot to include the maintainers with my first email.

Somewhere between Michael Lyle's original
"bcache: PI controller for writeback rate V2" patch dated 07 Sep 2017
and 1d316e65 bcache: implement PI controller for writeback rate,
the mapping of the writeback_rate_minimum attribute was dropped.

Re-add the missing sysfs writeback_rate_minimum attribute mapping to
"allow the user to specify a minimum rate at which dirty blocks are
retired."

Fixes: 1d316e65 ("bcache: implement PI controller for writeback rate")
Signed-off-by: Ben Peddell <klightspeed@killerwolves.net>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>