ext4: avoid divide by zero when trying to mount a corrupted file system

If s_log_groups_per_flex is greater than 31, then groups_per_flex will
will overflow and cause a divide by zero error.  This can cause kernel
BUG if such a file system is mounted.

Thanks to Nageswara R Sastry for analyzing the failure and providing
an initial patch.

http://bugzilla.kernel.org/show_bug.cgi?id=14287

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org