OpenZFS 5669 - altroot not set in zpool create

5669 altroot not set in zpool create when specified with -o
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/5669
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/c423721
Closes #4594

cmd/zpool/zpool_main.c

@@ -919,6 +919,8 @@ zpool_do_create(int argc, char **argv)
 					enable_all_pool_feat = B_FALSE;
 				}
 			}
+			if (zpool_name_to_prop(optarg) == ZPOOL_PROP_ALTROOT)
+				altroot = propval;
 			break;
 		case 'O':
 			if ((propval = strchr(optarg, '=')) == NULL) {