ext4: find the group descriptors on a 1k-block bigalloc,meta_bg filesystem

On a filesystem with a 1k block size, the group descriptors live in
block 2, not block 1.  If the filesystem has bigalloc,meta_bg set,
however, the calculation of the group descriptor table location does
not take this into account and returns the wrong block number.  Fix
the calculation to return the correct value for this case.

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