f2fs: use IS_INODE replace IS_DNODE in f2fs_flush_inline_data

Now IS_DNODE is used in f2fs_flush_inline_data and it has some problems:
1. Just only inodes may include inline data,not all direct nodes
2. When system IO is busy, it is inefficient to lock a direct node page
but not an inode page. Besides, if this direct node page is being
locked by others for IO, f2fs_flush_inline_data will be blocked here,
which will affects the checkpoint process, this is unreasonable.

So IS_INODE should be used in f2fs_flush_inline_data.

Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>