Bug fix to UFS/FFS superblock integrity checks when reading a superblock.

The original check verified that if an alternate superblock has not
been selected that the superblock is located in its standard location.
For UFS1 the with a 65536 block size, the first backup superblock
is at the same location as the UFS2 superblock. Since SBLOCK_UFS2
is the first location checked, the first backup is the superblock
that will be used for a UFS1 filesystems with a 65536 block size.
This patch allows the use of the first backup superblock in that
situation.

Reported by: Peter Holm
Tested by: Peter Holm
MFC after: 1 month (with 076002f24d35)
Differential Revision: https://reviews.freebsd.org/D35219