usb: gadget: f_fs: Fix kernel panic if use_os_string not set

If c->cdev->use_os_string flag is not set,
don't need to invoke ffs_do_os_descs() in _ffs_func_bind.
So uninitialized ext_compat_id pointer won't be accessed by
__ffs_func_bind_do_os_desc to cause kernel panic.
Signed-off-by: Jim Lin <jilin@nvidia.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_fs.c

@@ -2849,7 +2849,7 @@ static int _ffs_func_bind(struct usb_configuration *c,
 		goto error;
 
 	func->function.os_desc_table = vla_ptr(vlabuf, d, os_desc_table);
-	if (c->cdev->use_os_string)
+	if (c->cdev->use_os_string) {
 		for (i = 0; i < ffs->interfaces_count; ++i) {
 			struct usb_os_desc *desc;
 
@@ -2860,13 +2860,15 @@ static int _ffs_func_bind(struct usb_configuration *c,
 				vla_ptr(vlabuf, d, ext_compat) + i * 16;
 			INIT_LIST_HEAD(&desc->ext_prop);
 		}
-	ret = ffs_do_os_descs(ffs->ms_os_descs_count,
-			      vla_ptr(vlabuf, d, raw_descs) +
-			      fs_len + hs_len + ss_len,
-			      d_raw_descs__sz - fs_len - hs_len - ss_len,
-			      __ffs_func_bind_do_os_desc, func);
-	if (unlikely(ret < 0))
-		goto error;
+		ret = ffs_do_os_descs(ffs->ms_os_descs_count,
+				      vla_ptr(vlabuf, d, raw_descs) +
+				      fs_len + hs_len + ss_len,
+				      d_raw_descs__sz - fs_len - hs_len -
+				      ss_len,
+				      __ffs_func_bind_do_os_desc, func);
+		if (unlikely(ret < 0))
+			goto error;
+	}
 	func->function.os_desc_n =
 		c->cdev->use_os_string ? ffs->interfaces_count : 0;