signal/s390: Use force_sigsegv in default_trap_handler

Reading the history it is unclear why default_trap_handler calls
do_exit.  It is not even menthioned in the commit where the change
happened.  My best guess is that because it is unknown why the
exception happened it was desired to guarantee the process never
returned to userspace.

Using do_exit(SIGSEGV) has the problem that it will only terminate one
thread of a process, leaving the process in an undefined state.

Use force_sigsegv(SIGSEGV) instead which effectively has the same
behavior except that is uses the ordinary signal mechanism and
terminates all threads of a process and is generally well defined.

Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: linux-s390@vger.kernel.org
Fixes: ca2ab03237ec ("[PATCH] s390: core changes")
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Link: https://lkml.kernel.org/r/20211020174406.17889-11-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

arch/s390/kernel/traps.c

@@ -84,7 +84,7 @@ static void default_trap_handler(struct pt_regs *regs)
 {
 	if (user_mode(regs)) {
 		report_user_fault(regs, SIGSEGV, 0);
-		do_exit(SIGSEGV);
+		force_sigsegv(SIGSEGV);
 	} else
 		die(regs, "Unknown program exception");
 }