signal: Have the oom killer detect coredumps using signal->core_state

In preparation for removing the flag SIGNAL_GROUP_COREDUMP, change
__task_will_free_mem to test signal->core_state instead of the flag
SIGNAL_GROUP_COREDUMP.

Both fields are protected by siglock and both live in signal_struct so
there are no real tradeoffs here, just a change to which field is
being tested.

Link: https://lkml.kernel.org/r/20211213225350.27481-3-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

mm/oom_kill.c

@@ -793,7 +793,7 @@ static inline bool __task_will_free_mem(struct task_struct *task)
 	 * coredump_task_exit(), so the oom killer cannot assume that
 	 * the process will promptly exit and release memory.
 	 */
-	if (sig->flags & SIGNAL_GROUP_COREDUMP)
+	if (sig->core_state)
 		return false;
 
 	if (sig->flags & SIGNAL_GROUP_EXIT)