affs: use loff_t in affs_truncate()

It seems pretty unlikely that AFFS supports files over 4GB but we may as
well leave use loff_t just for cleanness sake instead of truncating it to
32 bits.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Marco Stornelli <marco.stornelli@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>