libbe(3): mount: the BE dataset is mounted at /

Other parts of libbe(3) were fairly strict on the mountpoint property of the
BE dataset, and be_mount was not much better. It was improved in r347027 to
allow mountpoint=none for depth==0, but this bit was still sensitive to
mountpoint != / and mountpoint != none. Given that other parts of libbe(3)
no longer restrict the mountpoint property here, and the rest of the base
system is generally OK and will assume that a BE is mounted at /, let's do
the same.

Reported by:	ler
MFC after:	3 days

lib/libbe/be_access.c

@@ -96,7 +96,7 @@ be_mount_iter(zfs_handle_t *zfs_hdl, void *data)
 	    NULL, NULL, 0, 1))
 		return (1);
 
-	if (strcmp("none", zfs_mnt) == 0) {
+	if (strcmp("none", zfs_mnt) == 0 || info->depth == 0) {
 		/*
 		 * mountpoint=none; we'll mount it at info->mountpoint assuming
 		 * we're at the root.  If we're not at the root, we're likely