blake3: fix up bogus checksums in face of cpu migration

This is a temporary measure until a better fix is sorted out.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Closes #14785
Closes #14808

module/zfs/blake3_zfs.c

@@ -50,7 +50,8 @@ abd_checksum_blake3_native(abd_t *abd, uint64_t size, const void *ctx_template,
 	ASSERT(ctx_template != NULL);
 
 #if defined(_KERNEL)
-	BLAKE3_CTX *ctx = blake3_per_cpu_ctx[CPU_SEQID_UNSTABLE];
+	kpreempt_disable();
+	BLAKE3_CTX *ctx = blake3_per_cpu_ctx[CPU_SEQID];
 #else
 	BLAKE3_CTX *ctx = kmem_alloc(sizeof (*ctx), KM_SLEEP);
 #endif
@@ -59,7 +60,9 @@ abd_checksum_blake3_native(abd_t *abd, uint64_t size, const void *ctx_template,
 	(void) abd_iterate_func(abd, 0, size, blake3_incremental, ctx);
 	Blake3_Final(ctx, (uint8_t *)zcp);
 
-#if !defined(_KERNEL)
+#if defined(_KERNEL)
+	kpreempt_enable();
+#else
 	memset(ctx, 0, sizeof (*ctx));
 	kmem_free(ctx, sizeof (*ctx));
 #endif