Skip spacemaps reading in case of pool readonly import

The only zdb utility require to read metaslab-related data during
read-only pool import because of spacemaps validation. Add global
variable which will allow zdb read spacemaps in case of readonly
import mode.

Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Fedor Uporov <fuporov.vstack@gmail.com>
Closes #9095
Closes #12687

cmd/zdb/zdb.c

@@ -118,6 +118,7 @@ extern int zfs_recover;
 extern unsigned long zfs_arc_meta_min, zfs_arc_meta_limit;
 extern int zfs_vdev_async_read_max_active;
 extern boolean_t spa_load_verify_dryrun;
+extern boolean_t spa_mode_readable_spacemaps;
 extern int zfs_reconstruct_indirect_combinations_max;
 extern int zfs_btree_verify_intensity;
 
@@ -8524,6 +8525,11 @@ main(int argc, char **argv)
 	 */
 	spa_load_verify_dryrun = B_TRUE;
 
+	/*
+	 * ZDB should have ability to read spacemaps.
+	 */
+	spa_mode_readable_spacemaps = B_TRUE;
+
 	kernel_init(SPA_MODE_READ);
 
 	if (dump_all)