f2fs: fix to force buffered IO on inline_data inode

It will return all zero data when DIO reading from inline_data inode, it
is because f2fs_iomap_begin() assign iomap->type w/ IOMAP_HOLE incorrectly
for this case.

We can let iomap framework handle inline data via assigning iomap->type
and iomap->inline_data correctly, however, it will be a little bit
complicated when handling race case in between direct IO and buffered IO.

So, let's force to use buffered IO to fix this issue.

Cc: stable@vger.kernel.org
Reported-by: Barry Song <v-songbaohua@oppo.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>