ext4: no need to truncate pagecache twice in collapse range

We're already calling truncate_pagecache() before we attempt to do any
actual job so there is not need to truncate pagecache once more using
truncate_setsize() after we're finished.

Remove truncate_setsize() and replace it just with i_size_write() note
that we're holding appropriate locks.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>