commit 829fa70d upstream.

A number of fuzzing failures seem to be caused by allocation bitmaps
or other metadata blocks being pointed at the superblock.

This can cause kernel BUG or WARNings once the superblock is
overwritten, so validate the group descriptor blocks to make sure this
doesn't happen.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>