Fix sign extension of bitmap_offset in super1.c

Fix sign extension of bitmap_offset in super1.c

fbdef49811c9e2b54e2064d9af68cfffa77c6e77 incorrectly tried to fix sign
extension of the bitmap offset. However mdinfo->bitmap_offset is a u32
and needs to be converted to a 32 bit signed integer before the sign
extension.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>