block/rnbd-clt: Fix missing a memory free when unloading the module

When unloading the rnbd-clt module, it does not free a memory
including the filename of the symbolic link to /sys/block/rnbdX.

It is found by kmemleak as below.

unreferenced object 0xffff9f1a83d3c740 (size 16):
  comm "bash", pid 736, jiffies 4295179665 (age 9841.310s)
  hex dump (first 16 bytes):
    21 64 65 76 21 6e 75 6c 6c 62 30 40 62 6c 61 00  !dev!nullb0@bla.
  backtrace:
    [<0000000039f0c55e>] 0xffffffffc0456c24
    [<000000001aab9513>] kernfs_fop_write+0xcf/0x1c0
    [<00000000db5aa4b3>] vfs_write+0xdb/0x1d0
    [<000000007a2e2207>] ksys_write+0x65/0xe0
    [<00000000055e280a>] do_syscall_64+0x50/0x1b0
    [<00000000c2b51831>] entry_SYSCALL_64_after_hwframe+0x49/0xbe
Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20210419073722.15351-13-gi-oh.kim@ionos.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rnbd/rnbd-clt-sysfs.c

@@ -432,10 +432,14 @@ void rnbd_clt_remove_dev_symlink(struct rnbd_clt_dev *dev)
 	 * i.e. rnbd_clt_unmap_dev_store() leading to a sysfs warning because
 	 * of sysfs link already was removed already.
 	 */
-	if (dev->blk_symlink_name && try_module_get(THIS_MODULE)) {
-		sysfs_remove_link(rnbd_devs_kobj, dev->blk_symlink_name);
+	if (dev->blk_symlink_name) {
+		if (try_module_get(THIS_MODULE)) {
+			sysfs_remove_link(rnbd_devs_kobj, dev->blk_symlink_name);
+			module_put(THIS_MODULE);
+		}
+		/* It should be freed always. */
 		kfree(dev->blk_symlink_name);
-		module_put(THIS_MODULE);
+		dev->blk_symlink_name = NULL;
 	}
 }