G/C data_alloc_arena

It is a leftover from illumos always set to NULL and introducing a
spurious difference between zio_buf and zio_data_buf.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Mateusz Guzik <[email protected]>
Closes openzfs#11188

Author: mjguzik

module/zfs/zio.c

@@ -144,7 +144,6 @@ void
 zio_init(void)
 {
 	size_t c;
-	vmem_t *data_alloc_arena = NULL;
 
 	zio_cache = kmem_cache_create("zio_cache",
 	    sizeof (zio_t), 0, NULL, NULL, NULL, NULL, NULL, 0);
@@ -213,8 +212,7 @@ zio_init(void)
 			(void) snprintf(name, sizeof (name), "zio_data_buf_%lu",
 			    (ulong_t)size);
 			zio_data_buf_cache[c] = kmem_cache_create(name, size,
-			    align, NULL, NULL, NULL, NULL,
-			    data_alloc_arena, data_cflags);
+			    align, NULL, NULL, NULL, NULL, NULL, data_cflags);
 		}
 	}