ext4: fix setup_new_group_blocks locking

setup_new_group_blocks() manipulates the group descriptor block bh
under the block_bitmap bh's lock.  It shouldn't matter since nobody
but resize should be touching these blocks, but it's worth fixing up.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>