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>

drivers/md/bcache/sysfs.c

@@ -285,6 +285,7 @@ STORE(__cached_dev)
 			    1, WRITEBACK_RATE_UPDATE_SECS_MAX);
 	d_strtoul(writeback_rate_i_term_inverse);
 	d_strtoul_nonzero(writeback_rate_p_term_inverse);
+	d_strtoul_nonzero(writeback_rate_minimum);
 
 	sysfs_strtoul_clamp(io_error_limit, dc->error_limit, 0, INT_MAX);
 
@@ -412,6 +413,7 @@ static struct attribute *bch_cached_dev_files[] = {
 	&sysfs_writeback_rate_update_seconds,
 	&sysfs_writeback_rate_i_term_inverse,
 	&sysfs_writeback_rate_p_term_inverse,
+	&sysfs_writeback_rate_minimum,
 	&sysfs_writeback_rate_debug,
 	&sysfs_errors,
 	&sysfs_io_error_limit,