bcache: fix error setting writeback_rate through sysfs interface

Commit ea8c535 ("bcache: set max writeback rate when I/O request
is idle") changes struct bch_ratelimit member rate from uint32_t to
atomic_long_t and uses atomic_long_set() in drivers/md/bcache/sysfs.c
to set new writeback rate, after the input is converted from memory
buf to long int by sysfs_strtoul_clamp().

The above change has a problem because there is an implicit return
inside sysfs_strtoul_clamp() so the following atomic_long_set()
won't be called. This error is detected by 0day system with following
snipped smatch warnings:

drivers/md/bcache/sysfs.c:271 __cached_dev_store() error: uninitialized
symbol 'v'.
270  sysfs_strtoul_clamp(writeback_rate, v, 1, INT_MAX);
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@271 atomic_long_set(&dc->writeback_rate.rate, v);

This patch fixes the above error by using strtoul_safe_clamp() to
convert the input buffer into a long int type result.

Fixes: ea8c535 ("bcache: set max writeback rate when I/O request is idle")
Cc: Kai Krakow <kai@kaishome.de>
Cc: Stefan Priebe <s.priebe@profihost.ag>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/sysfs.c

@@ -265,10 +265,17 @@ STORE(__cached_dev)
 	sysfs_strtoul_clamp(writeback_percent, dc->writeback_percent, 0, 40);
 
 	if (attr == &sysfs_writeback_rate) {
-		int v;
+		ssize_t ret;
+		long int v = atomic_long_read(&dc->writeback_rate.rate);
+
+		ret = strtoul_safe_clamp(buf, v, 1, INT_MAX);
 
-		sysfs_strtoul_clamp(writeback_rate, v, 1, INT_MAX);
-		atomic_long_set(&dc->writeback_rate.rate, v);
+		if (!ret) {
+			atomic_long_set(&dc->writeback_rate.rate, v);
+			ret = size;
+		}
+
+		return ret;
 	}
 
 	sysfs_strtoul_clamp(writeback_rate_update_seconds,