ext4: split ext4_ind_truncate from ext4_truncate

We are about to move all indirect inode functions to a new file.
Before we do that, let's split ext4_ind_truncate() out of ext4_truncate()
leaving only generic code in the latter, so we will be able to move
ext4_ind_truncate() to the new file.

Signed-off-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>