ext4: Prepare API change for files greater than 2GB

Change the internal EXT4 functions to use loff_t for offsets.

Signed-off-by: Suriyan Ramasami <suriyan.r@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
[trini: Update common/spl/spl_ext.c]
Signed-off-by: Tom Rini <trini@ti.com>