ARM: alignment: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

arch/arm/mm/alignment.c: In function 'thumb2arm':
arch/arm/mm/alignment.c:688:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if ((tinstr & (3 << 9)) == 0x0400) {
      ^
arch/arm/mm/alignment.c:700:2: note: here
  default:
  ^~~~~~~
arch/arm/mm/alignment.c: In function 'do_alignment_t32_to_handler':
arch/arm/mm/alignment.c:753:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
   poffset->un = (tinst2 & 0xff) << 2;
   ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
arch/arm/mm/alignment.c:754:2: note: here
  case 0xe940:
  ^~~~
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

arch/arm/mm/alignment.c

@@ -695,7 +695,7 @@ thumb2arm(u16 tinstr)
 			return subset[(L<<1) | ((tinstr & (1<<8)) >> 8)] |
 			    (tinstr & 255);		/* register_list */
 		}
-		/* Else fall through for illegal instruction case */
+		/* Else, fall through - for illegal instruction case */
 
 	default:
 		return BAD_INSTR;
@@ -751,6 +751,8 @@ do_alignment_t32_to_handler(unsigned long *pinstr, struct pt_regs *regs,
 	case 0xe8e0:
 	case 0xe9e0:
 		poffset->un = (tinst2 & 0xff) << 2;
+		/* Fall through */
+
 	case 0xe940:
 	case 0xe9c0:
 		return do_alignment_ldrdstrd;