treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

security/selinux/ss/mls.c

@@ -535,7 +535,7 @@ int mls_compute_sid(struct policydb *p,
 						  scontext, tcontext);
 		}
 
-		/* Fallthrough */
+		fallthrough;
 	case AVTAB_CHANGE:
 		if ((tclass == p->process_class) || sock)
 			/* Use the process MLS attributes. */
@@ -546,8 +546,6 @@ int mls_compute_sid(struct policydb *p,
 	case AVTAB_MEMBER:
 		/* Use the process effective MLS attributes. */
 		return mls_context_cpy_low(newcontext, scontext);
-
-	/* fall through */
 	}
 	return -EINVAL;
 }