f2fs: fix to truncate inline data past EOF

Previously if inode is with inline data, we will try to invalid partial inline
data in page #0 when we truncate size of inode in truncate_partial_data_page().
And then we set page #0 to dirty, after this we can synchronize inode page with
page #0 at ->writepage().

But sometimes we will fail to operate page #0 in truncate_partial_data_page()
due to below reason:
a) if offset is zero, we will skip setting page #0 to dirty.
b) if page #0 is not uptodate, we will fail to update it as it has no mapping
data.

So with following operations, we will meet recent data which should be
truncated.

1.write inline data to file
2.sync first data page to inode page
3.truncate file size to 0
4.truncate file size to max_inline_size
5.echo 1 > /proc/sys/vm/drop_caches
6.read file --> meet original inline data which is remained in inode page.

This patch renames truncate_inline_data() to truncate_inline_inode() for code
readability, then use truncate_inline_inode() to truncate inline data in inode
page in truncate_blocks() and truncate page #0 in truncate_partial_data_page()
for fixing.

v2:
 o truncate partially #0 page in truncate_partial_data_page to avoid keeping
   old data in #0 page.

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