ext3, ext4: avoid divide by zero

As it turns out, the kernel divides by EXT3_INODES_PER_GROUP(s) when
mounting an ext3 filesystem.  If that number is zero, a crash follows.
Below a patch.

This crash was reported by Joeri de Ruiter, Carst Tankink and Pim Vullers.

Cc: <linux-ext4@vger.kernel.org>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>