f2fs: fix to truncate inline data in inode page when setattr

Previous we do not truncate inline data in inode page when setattr, so following
case could still read the inline data which has already truncated:

1.write inline data
2.ftruncate size to 0
3.ftruncate size to max inline data size
4.read from offset 0

This patch introduces truncate_inline_data() to fix this problem.

change log from v1:
 o fix a bug and do not truncate first page data after truncate inline data.

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