A race condition existed between the time a UFS/FFS superblock check
hash was computed and the time that the superblock was copied to a
buffer to be written to disk. The result was a failed superblock
check hash the next time that the superblock was read.

The fix is to compute the check hash after the superblock has been
copied to a buffer to be written.

PR: 236504
Reported by: Peter Holm
Tested by: Peter Holm
Sponsored by: Netflix