Fix automount for root filesystems

Commit 093bb64 resolved an automount failures for chroot'd processes
but inadvertently broke automounting for root filesystems where the
vfs_mntpoint is NULL.  Resolve the issue by checking for NULL in order
to generate the correct path.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#9381
Closes openzfs#9384

module/zfs/zfs_ctldir.c

@@ -1053,7 +1053,8 @@ zfsctl_snapshot_mount(struct path *path, int flags)
 	 * on mount.zfs(8).
 	 */
 	snprintf(full_path, MAXPATHLEN, "%s/.zfs/snapshot/%s",
-	    zfsvfs->z_vfs->vfs_mntpoint, dname(dentry));
+	    zfsvfs->z_vfs->vfs_mntpoint ? zfsvfs->z_vfs->vfs_mntpoint : "",
+	    dname(dentry));
 
 	/*
 	 * Multiple concurrent automounts of a snapshot are never allowed.