md-cluster: Fix potential error pointer dereference in resize_bitmaps()

The error handling calls md_bitmap_free(bitmap) which checks for NULL
but will Oops if we pass an error pointer.  Let's set "bitmap" to NULL
on this error path.

Fixes: afd75628 ("md-cluster/raid10: resize all the bitmaps before start reshape")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>