Fix error handling for "zpool online -e".

The error handling code around zpool_relabel_disk() is either inexistent
or wrong. The function call itself is not checked, and
zpool_relabel_disk() is generating error messages from an unitialized
buffer.

Before:

    # zpool online -e homez sdb; echo $?
    `: cannot relabel 'sdb1': unable to open device: 2
    0

After:

    # zpool online -e homez sdb; echo $?
    cannot expand sdb: cannot relabel 'sdb1': unable to open device: 2
    1

lib/libzfs/libzfs_pool.c

@@ -2081,15 +2081,14 @@ zpool_get_physpath(zpool_handle_t *zhp, char *physpath, size_t phypath_size)
  * the disk to use the new unallocated space.
  */
 static int
-zpool_relabel_disk(libzfs_handle_t *hdl, const char *path)
+zpool_relabel_disk(libzfs_handle_t *hdl, const char *path, const char *msg)
 {
-	char errbuf[1024];
 	int fd, error;
 
 	if ((fd = open(path, O_RDWR|O_DIRECT)) < 0) {
 		zfs_error_aux(hdl, dgettext(TEXT_DOMAIN, "cannot "
 		    "relabel '%s': unable to open device: %d"), path, errno);
-		return (zfs_error(hdl, EZFS_OPENFAILED, errbuf));
+		return (zfs_error(hdl, EZFS_OPENFAILED, msg));
 	}
 
 	/*
@@ -2102,7 +2101,7 @@ zpool_relabel_disk(libzfs_handle_t *hdl, const char *path)
 	if (error && error != VT_ENOSPC) {
 		zfs_error_aux(hdl, dgettext(TEXT_DOMAIN, "cannot "
 		    "relabel '%s': unable to read disk capacity"), path);
-		return (zfs_error(hdl, EZFS_NOCAP, errbuf));
+		return (zfs_error(hdl, EZFS_NOCAP, msg));
 	}
 	return (0);
 }
@@ -2160,7 +2159,9 @@ zpool_vdev_online(zpool_handle_t *zhp, const char *path, int flags,
 
 		if (wholedisk) {
 			pathname += strlen(DISK_ROOT) + 1;
-			(void) zpool_relabel_disk(hdl, pathname);
+			int result = zpool_relabel_disk(hdl, pathname, msg);
+			if (result != 0)
+				return (result);
 		}
 	}