Reflect __{start,end}_init_task kernel symbols rename

Kernel commit 8f69cba096b5 ("x86: Rename __{start,end}_init_task to
__{start,end}_init_stack") leads to failure when crash loading:

	crash: invalid count request: 0

Assume both __{start,end}_init_task and __{start,end}_init_stack
symbols could exist for backward compatibility.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

task.c

@@ -496,10 +496,17 @@ task_init(void)
 	    	((len = SIZE(thread_union)) != STACKSIZE())) {
 		machdep->stacksize = len;
 	} else if (!VALID_SIZE(thread_union) && !VALID_SIZE(task_union)) {
-		if (kernel_symbol_exists("__start_init_task") &&
-		    kernel_symbol_exists("__end_init_task")) {
+		len = 0;
+		if (kernel_symbol_exists("__start_init_stack") &&
+		   kernel_symbol_exists("__end_init_stack")) {
+			len = symbol_value("__end_init_stack");
+			len -= symbol_value("__start_init_stack");
+		} else if (kernel_symbol_exists("__start_init_task") &&
+			kernel_symbol_exists("__end_init_task")) {
 			len = symbol_value("__end_init_task");
 			len -= symbol_value("__start_init_task");
+		}
+		if (len) {
 			ASSIGN_SIZE(thread_union) = len;
 			machdep->stacksize = len;
 		}