f2fs: fix to truncate blocks past EOF in ->setattr

f2fs: fix to truncate blocks past EOF in ->setattr

By using FALLOC_FL_KEEP_SIZE in ->fallocate of f2fs, we can fallocate block past
EOF without changing i_size of inode. These blocks past EOF will not be
truncated in ->setattr as we truncate them only when change the file size.

We should give a chance to truncate blocks out of filesize in setattr().

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>