ext4: Add sanity checks for the superblock before mounting the filesystem

This avoids insane superblock configurations that could lead to kernel
oops due to null pointer derefences.

http://bugzilla.kernel.org/show_bug.cgi?id=12371

Thanks to David Maciejak at Fortinet's FortiGuard Global Security
Research Team who discovered this bug independently (but at
approximately the same time) as Thiemo Nagel, who submitted the patch.

Signed-off-by: Thiemo Nagel <thiemo.nagel@ph.tum.de>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org