Add separate field for indicating that spa is in middle of split

By default it's not possible to open a device already owned by an
active vdev. It's necessary to make an exception to this for vdev
split. The FreeBSD platform code will make an exception if
spa_is splitting is set to to true.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10178

module/zfs/spa.c

@@ -7477,6 +7477,7 @@ spa_vdev_split_mirror(spa_t *spa, char *newname, nvlist_t *config,
 	list_destroy(&vd_trim_list);
 
 	newspa->spa_config_source = SPA_CONFIG_SRC_SPLIT;
+	newspa->spa_is_splitting = B_TRUE;
 
 	/* create the new pool from the disks of the original pool */
 	error = spa_load(newspa, SPA_LOAD_IMPORT, SPA_IMPORT_ASSEMBLE);
@@ -7554,6 +7555,7 @@ spa_vdev_split_mirror(spa_t *spa, char *newname, nvlist_t *config,
 	spa_history_log_internal(newspa, "split", NULL,
 	    "from pool %s", spa_name(spa));
 
+	newspa->spa_is_splitting = B_FALSE;
 	kmem_free(vml, children * sizeof (vdev_t *));
 
 	/* if we're not going to mount the filesystems in userland, export */