Create: Block automatic enabling bitmap for external metadata

For external metadata, bitmap should be added only when
explicitly set by the administrator.
They could be additional requirements to consider before
enabling the external metadata's functionality
(e.g., kernel support).

Signed-off-by: Jakub Radtke <jakub.radtke@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>

Create.c

@@ -540,6 +540,7 @@ int Create(struct supertype *st, char *mddev,
 	}
 
 	if (!s->bitmap_file &&
+	    !st->ss->external &&
 	    s->level >= 1 &&
 	    st->ss->add_internal_bitmap &&
 	    s->journaldisks == 0 &&