sh: Fix sigmask trampling in signal delivery.

There was a missing return in do_signal() that caused the saved
sigmask to be written back after having successfully delivered
the signal.
Signed-off-by: NRyusuke Sakato <sakato@hsdv.com>
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

arch/sh/kernel/signal.c

@@ -589,6 +589,8 @@ static void do_signal(struct pt_regs *regs, unsigned int save_r0)
 			if (test_thread_flag(TIF_RESTORE_SIGMASK))
 				clear_thread_flag(TIF_RESTORE_SIGMASK);
 		}
+
+		return;
 	}
 
  no_signal: