Fix AVX512BW Fletcher code on AVX512-but-not-BW machines

Introduce a specific valid function for avx512f+avx512bw (instead 
of checking only for avx512f).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Adam Moss <c@yotes.com>
Signed-off-by: Romain Dolbeau <romain@dolbeau.org>
Closes openzfs#11937
Closes openzfs#11938

module/zcommon/zfs_fletcher_avx512.c

@@ -210,14 +210,20 @@ fletcher_4_avx512bw_byteswap(fletcher_4_ctx_t *ctx, const void *buf,
 }
 STACK_FRAME_NON_STANDARD(fletcher_4_avx512bw_byteswap);
 
+static boolean_t
+fletcher_4_avx512bw_valid(void)
+{
+	return (fletcher_4_avx512f_valid() && zfs_avx512bw_available());
+}
+
 const fletcher_4_ops_t fletcher_4_avx512bw_ops = {
 	.init_native = fletcher_4_avx512f_init,
 	.fini_native = fletcher_4_avx512f_fini,
 	.compute_native = fletcher_4_avx512f_native,
 	.init_byteswap = fletcher_4_avx512f_init,
 	.fini_byteswap = fletcher_4_avx512f_fini,
 	.compute_byteswap = fletcher_4_avx512bw_byteswap,
-	.valid = fletcher_4_avx512f_valid,
+	.valid = fletcher_4_avx512bw_valid,
 	.name = "avx512bw"
 };
 #endif