f2fs: fix possible data corruption in f2fs_write_begin()

f2fs: fix possible data corruption in f2fs_write_begin()

commit 9234f3190bf8b25b11b105191d408ac50a107948 upstream.

f2fs_write_begin() doesn't initialize the 'dn' variable if the inode has
inline data. However it uses its contents to decide whether it should
just zero out the page or load data to it. Thus if we are unlucky we can
zero out page contents instead of loading inline data into a page.

CC: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>