spa: Fix FreeBSD sysctl handlers

sbuf_cpy() resets the sbuf state, which is wrong for sbufs allocated by
sbuf_new_for_sysctl().  In particular, this code triggers an assertion
failure in sbuf_clear().

Instead, allocate the sbuf itself statically and pass the buffer in at
creation time.  This avoids some needless memory allocation and copying.

Fixes: 6930ecb ("spa: make read/write queues configurable")
Reported-by: Peter Holm <[email protected]>
Signed-off-by: Mark Johnston <[email protected]>

module/zfs/spa.c

@@ -1460,11 +1460,12 @@ spa_taskq_read_param(ZFS_MODULE_PARAM_ARGS)
 	int err = 0;
 
 	if (req->newptr == NULL) {
+		struct sbuf s;
 		int len = spa_taskq_param_get(ZIO_TYPE_READ, buf);
-		struct sbuf *s = sbuf_new_for_sysctl(NULL, NULL, len+1, req);
-		sbuf_cpy(s, buf);
-		err = sbuf_finish(s);
-		sbuf_delete(s);
+
+		(void) sbuf_new_for_sysctl(&s, buf, len + 1, req);
+		err = sbuf_finish(&s);
+		sbuf_delete(&s);
 		return (err);
 	}
 
@@ -1481,11 +1482,12 @@ spa_taskq_write_param(ZFS_MODULE_PARAM_ARGS)
 	int err = 0;
 
 	if (req->newptr == NULL) {
+		struct sbuf s;
 		int len = spa_taskq_param_get(ZIO_TYPE_WRITE, buf);
-		struct sbuf *s = sbuf_new_for_sysctl(NULL, NULL, len+1, req);
-		sbuf_cpy(s, buf);
-		err = sbuf_finish(s);
-		sbuf_delete(s);
+
+		(void) sbuf_new_for_sysctl(&s, buf, len + 1, req);
+		err = sbuf_finish(&s);
+		sbuf_delete(&s);
 		return (err);
 	}