ext4: Fix big-endian problem in __ext4_check_blockref()

Commit fe2c8191 introduced a regression on big-endian system, because
the checks to make sure block references in non-extent inodes are
valid failed to use le32_to_cpu().

Reported-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Thiemo Nagel <thiemo.nagel@ph.tum.de>
Tested-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org