signal/sh: Use force_sig(SIGKILL) instead of do_group_exit(SIGKILL)

Today the sh code allocates memory the first time a process uses
the fpu.  If that memory allocation fails, kill the affected task
with force_sig(SIGKILL) rather than do_group_exit(SIGKILL).

Calling do_group_exit from an exception handler can potentially lead
to dead locks as do_group_exit is not designed to be called from
interrupt context.  Instead use force_sig(SIGKILL) to kill the
userspace process.  Sending signals in general and force_sig in
particular has been tested from interrupt context so there should be
no problems.

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: linux-sh@vger.kernel.org
Fixes: 0ea820cf ("sh: Move over to dynamically allocated FPU context.")
Link: https://lkml.kernel.org/r/20211020174406.17889-6-ebiederm@xmission.comSigned-off-by: Eric W. Biederman <ebiederm@xmission.com>

arch/sh/kernel/cpu/fpu.c

@@ -62,18 +62,20 @@ void fpu_state_restore(struct pt_regs *regs)
 	}
 
 	if (!tsk_used_math(tsk)) {
-		local_irq_enable();
+		int ret;
 		/*
 		 * does a slab alloc which can sleep
 		 */
-		if (init_fpu(tsk)) {
+		local_irq_enable();
+		ret = init_fpu(tsk);
+		local_irq_disable();
+		if (ret) {
 			/*
 			 * ran out of memory!
 			 */
-			do_group_exit(SIGKILL);
+			force_sig(SIGKILL);
 			return;
 		}
-		local_irq_disable();
 	}
 
 	grab_fpu(regs);