ext3: 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>
C: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>