ext4: avoid clearing beyond i_blocks when truncating an inline data file

A missing cast means that when we are truncating a file which is less
than 60 bytes, we don't clear the correct area of memory, and in fact
we can end up truncating the next inode in the inode table, or worse
yet, some other kernel data structure.

Addresses-Coverity-Id: #751987

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