super1: Clear memory allocated for superblock + bitmap before use

super1: Clear memory allocated for superblock + bitmap before use

load_super1() did not clear memory allocated for the superblock +
bitmap. This causes issues if the superblock does not contain a bitmap
as later checks of bitmap features would rely on the bits being
cleared.

This bug has been around for a long time, but was only exposed in
mdadm-3.4 with the introduction of the clustering code.

Reported-by: Jan Stodola <jstodola@redhat.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>