m68k: Missing syscall_trace() on sigreturn

If we leave sigreturn via ret_from_signal, we end up with syscall
trace only on entry, leading to very unhappy strace, among other
things.  Note that this means different behaviours for signals
delivered while we were in pagefault and for ones delivered while
we were in interrupt...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>