f2fs: remove useless truncate in f2fs_collapse_range()

Since offset < new_size, no need to do truncate_pagecache() again
with new_size.

Signed-off-by: Wei Fang <fangwei1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>