libext2fs: don't always read backup group descriptors on a 1k-block meta_bg fs

libext2fs: don't always read backup group descriptors on a 1k-block meta_bg fs

On a filesystem with 1K blocks and meta_bg enabled, opening a
filesystem with automatic superblock detection tries to compensate for
the fact that the superblock lives in block 1.  However, the method by
which this is done is later misinterpreted to mean "read the backup
group descriptors", which is not what we want in this case.

Therefore, in ext2fs_open3() separate the 'group zero' adjustment into
its own variable so that we don't get fed backup group descriptors
when we try to load meta_bg group descriptors.

Furthermore, enhance ext2fs_descriptor_block_loc2() to perform its own
group zero correction.  The other caller of this function neglects to
do any group-zero correction of their own, so this fixes them too.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>