Ext4: Fix online resize block group descriptor corruption

This is the patch for the group descriptor table corruption during
online resize pointed out by Theodore Tso.  The problem was caused by
the fact that the ext4 group descriptor can be either 32 or 64 bytes
long.  Only the 64 bytes structure was taken into account.

Signed-off-by: Frederic Bohe <frederic.bohe@bull.net>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>