Fix memory allocation issue for BLAKE3 context

The kmem_alloc(sizeof (*ctx), KM_NOSLEEP) call on FreeBSD can't be
used in this code segment. Work around this by pre-allocating a percpu
context array for later use.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13568

module/zfs/zfs_chksum.c

@@ -277,6 +277,9 @@ chksum_benchmark(void)
 void
 chksum_init(void)
 {
+#ifdef _KERNEL
+	blake3_per_cpu_ctx_init();
+#endif
 
 	/* Benchmark supported implementations */
 	chksum_benchmark();
@@ -313,4 +316,8 @@ chksum_fini(void)
 		chksum_stat_cnt = 0;
 		chksum_stat_data = 0;
 	}
+
+#ifdef _KERNEL
+	blake3_per_cpu_ctx_fini();
+#endif
 }