ext4: use i_size_read in ext4_unaligned_aio()

We haven't taken i_mutex yet, so we need to use i_size_read().

Change-Id: Ib04d010106b7ecb9b264d93baceefaf49b84f779
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org