a mismatch, but will allow fsck to continue when the last alternate
superblock gets corrupted somehow.
Also, remove searching for alternate super blocks. It should have been
removed two years ago with r276737 by imp@. Leave minor vestiges in
place in case someone wants to solve the hard problem of knowing where
altnernate superblocks live without access to data formerly stored in
disklabels.
Differential Revision: https://reviews.freebsd.org/D11589