md-cluster/bitmap: don't call md_bitmap_sync_with_cluster during reshaping stage

When reshape is happening in one node, other nodes could receive
lots of RESYNCING messages, so md_bitmap_sync_with_cluster is called.

Since the resyncing window is typically small in these RESYNCING
messages, so WARN is always triggered, so we should not call the
func when reshape is happening.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>