openrisc: restore all regs on rt_sigreturn

Fix signal handling for when signals are handled as the result of timers
or exceptions, previous code assumed syscalls. This was noticeable with X
crashing where it uses SIGALRM.

This patch restores all regs before returning to userspace via
_resume_userspace instead of via syscall return path.

The rt_sigreturn syscall is more like a context switch than a function
call; it entails a return from one context (the signal handler) to another
(the process in question).  For a context switch like this there are
effectively no call-saved regs that remain constant across the transition.

Reported-by: Sebastian Macke <sebastian@macke.de>
Signed-off-by: Jonas Bonn <jonas@southpole.se>
Tested-by: Guenter Roeck <linux@roeck-us.net>
[shorne@gmail.com: Updated comment better reflect change and issue]
Signed-off-by: Stafford Horne <shorne@gmail.com>

arch/openrisc/kernel/entry.S

@@ -1101,8 +1101,16 @@ ENTRY(__sys_fork)
 	 l.addi	r3,r1,0
 
 ENTRY(sys_rt_sigreturn)
-	l.j	_sys_rt_sigreturn
+	l.jal	_sys_rt_sigreturn
 	 l.addi	r3,r1,0
+	l.sfne	r30,r0
+	l.bnf	_no_syscall_trace
+	 l.nop
+	l.jal	do_syscall_trace_leave
+	 l.addi	r3,r1,0
+_no_syscall_trace:
+	l.j	_resume_userspace
+	 l.nop
 
 /* This is a catch-all syscall for atomic instructions for the OpenRISC 1000.
  * The functions takes a variable number of parameters depending on which