ARM: signal: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning:

arch/arm/kernel/signal.c: In function 'do_signal':
arch/arm/kernel/signal.c:598:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
restart -= 2;
~~~~~~~~^~~~
arch/arm/kernel/signal.c:599:3: note: here
case -ERESTARTNOHAND:
^~~~

Reported-by: Stephen Rothwell <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>

arch/arm/kernel/signal.c

···
 		switch (retval) {
 		case -ERESTART_RESTARTBLOCK:
 			restart -= 2;
+			/* Fall through */
 		case -ERESTARTNOHAND:
 		case -ERESTARTSYS:
 		case -ERESTARTNOINTR: