The binary representation of the superblock (the fs structure) is written
out verbatim to the disk: see ffs_sbput() in sys/ufs/ffs/ffs_subr.c.
It contains a pointer to the fs_summary_info structure. This pointer
value inadvertently causes garbage to be stored. It is garbage because
the pointer to the fs_summary_info structure is the address the then
current stack or heap. Although a mere pointer does not reveal anything
useful (like a part of a private key) to an attacker, garbage output
deteriorates reproducibility.

This commit zeros out the pointer to the fs_summary_info structure
before writing the out the superblock.

Reviewed by: kib
Tested by: Peter Holm
PR: 246983
Sponsored by: Netflix