Make clang happy with vdev_raidz_ code

The macros are used to generate code for conditions without a
corresponding branch. This is not a problem in practice, but
clang has no way of knowing that. Add a default branch with a
VERIFY(0) to indicate that it "can't happen"

```
In file included from \
/usr/home/mmacy/devel/ZoF/module/zfs/vdev_raidz_math_sse2.c:607:
/usr/home/mmacy/devel/ZoF/module/zfs/vdev_raidz_math_impl.h:281:3: \
error: no case matching constant switch condition '3' [-Werror]
```

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9434

module/zfs/vdev_raidz_math_avx512f.c

@@ -29,6 +29,7 @@
 
 #include <sys/types.h>
 #include <sys/simd.h>
+#include <sys/debug.h>
 
 #define	__asm __asm__ __volatile__
 
@@ -194,6 +195,8 @@ typedef struct v {
 		    "vpternlogd $0x6c,%zmm29, %zmm26, %" VR0(r) "\n"	\
 		    "vpternlogd $0x6c,%zmm29, %zmm25, %" VR1(r));	\
 		break;							\
+	default:							\
+		VERIFY(0);						\
 	}								\
 }
 
@@ -370,6 +373,9 @@ gf_x2_mul_fns[256] = {
 		COPY(R_23(r), _mul_x2_in);				\
 		gf_x2_mul_fns[c]();					\
 		COPY(_mul_x2_acc, R_23(r));				\
+		break;							\
+	default:							\
+		VERIFY(0);						\
 	}								\
 }
 

module/zfs/vdev_raidz_math_sse2.c

@@ -28,6 +28,7 @@
 
 #include <sys/types.h>
 #include <sys/simd.h>
+#include <sys/debug.h>
 
 #define	__asm __asm__ __volatile__
 
@@ -125,6 +126,8 @@ typedef struct v {
 		__asm(							\
 		    "movdqa %" VR0(r) ", %" VR1(r));			\
 		break;							\
+	default:							\
+		VERIFY(0);						\
 	}								\
 }
 
@@ -175,6 +178,8 @@ typedef struct v {
 		    "movdqa %%" VR0(r)", 0x00(%[DST])\n"		\
 		    : : [DST] "r" (dst));				\
 		break;							\
+	default:							\
+		VERIFY(0);						\
 	}								\
 }
 
@@ -508,6 +513,8 @@ gf_x2_mul_fns[256] = {
 		gf_x1_mul_fns[c]();					\
 		COPY(_mul_x1_acc, r);					\
 		break;							\
+	default:							\
+		VERIFY(0);						\
 	}								\
 }