While malloc sleeps, the disk list could change during sysctl. Then

While malloc sleeps, the disk list could change during sysctl.  Then
allocated memory could be too short for the list of disks.  Retry
allocating enough space until it did not change.
The disk list and duid memory are protected by kernel lock.  Use
asserts to mark this explicitly.
Reported-by: syzbot+807423f6868bbfb836bc@syzkaller.appspotmail.com
OK anton@ mpi@