m68k: Update ->thread.esp0 before calling syscall_trace() in ret_from_signal

We get there when sigreturn has performed obscene acts on kernel stack;
in particular, the location of pt_regs has shifted.  We are about to call
syscall_trace(), which might stop for tracer.  If that happens, we'd better
have task_pt_regs() returning correct result...
Fucked-up-by: NAl Viro <viro@zeniv.linux.org.uk>
Fixes: bd6f56a7 ("m68k: Missing syscall_trace() on sigreturn")
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>
Tested-by: NMichael Schmitz <schmitzmic@gmail.com>
Reviewed-by: NMichael Schmitz <schmitzmic@gmail.com>
Tested-by: NFinn Thain <fthain@linux-m68k.org>
Link: https://lore.kernel.org/r/YP2dMWeV1LkHiOpr@zeniv-ca.linux.org.ukSigned-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/kernel/entry.S

@@ -186,6 +186,8 @@ ENTRY(ret_from_signal)
 	movel	%curptr@(TASK_STACK),%a1
 	tstb	%a1@(TINFO_FLAGS+2)
 	jge	1f
+	lea	%sp@(SWITCH_STACK_SIZE),%a1
+	movel	%a1,%curptr@(TASK_THREAD+THREAD_ESP0)
 	jbsr	syscall_trace
 1:	RESTORE_SWITCH_STACK
 	addql	#4,%sp