Write out corrected superblock when creating a UFS/FFS snapshot.

When taking a snapshot on a UFS version 1 filesystem we need to
call ffs_oldfscompat_write() to unwind any in-memory changes that
were made to the superblock before writing it. The cause of this bug
was that the trimmed down maximum file size was not being reverted.

PR: 271352
Tested-by: Peter Holm
MFC-after: 1 week
Sponsored-by: The FreeBSD Foundation