RDMA/rnbd: Fix compilation error when CONFIG_MODULES is disabled

module_is_live function is only defined when CONFIG_MODULES is enabled.
Use try_module_get instead to check whether the module is being removed.

When module unload and manuall unmapping is happening in parallel, we can
try removing the symlink twice: rnbd_client_exit
vs. rnbd_clt_unmap_dev_store.

This is probably not the best way to deal with this race in general, but
for now this fixes the compilation issue when CONFIG_MODULES is disabled
and has no functional impact. Regression tests passed.

Fixes: 1eb54f8f ("block/rnbd: client: sysfs interface functions")
Link: https://lore.kernel.org/r/20200521185909.457245-1-danil.kipnis@cloud.ionos.comReported-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

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

@@ -428,12 +428,15 @@ static struct attribute *rnbd_dev_attrs[] = {
 void rnbd_clt_remove_dev_symlink(struct rnbd_clt_dev *dev)
 {
 	/*
-	 * The module_is_live() check is crucial and helps to avoid annoying
-	 * sysfs warning raised in sysfs_remove_link(), when the whole sysfs
-	 * path was just removed, see rnbd_close_sessions().
+	 * The module unload rnbd_client_exit path is racing with unmapping of
+	 * the last single device from the sysfs manually
+	 * i.e. rnbd_clt_unmap_dev_store() leading to a sysfs warning because
+	 * of sysfs link already was removed already.
 	 */
-	if (strlen(dev->blk_symlink_name) && module_is_live(THIS_MODULE))
+	if (strlen(dev->blk_symlink_name) && try_module_get(THIS_MODULE)) {
 		sysfs_remove_link(rnbd_devs_kobj, dev->blk_symlink_name);
+		module_put(THIS_MODULE);
+	}
 }
 
 static struct kobj_type rnbd_dev_ktype = {