ext4: fix online resize bug

There is a bug when we are trying to verify that the reserve inode's
double indirect blocks point back to the primary gdt blocks.  The fix is
obvious, we need to mod the gdb count by the addr's per block.  This was
verified using the same testcase as with the ext3 equivalent of this
patch.

Signed-off-by: Josef Bacik <jbacik@redhat.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>