f2fs: fix to set ipu policy

For LFS mode, it should update outplace and no need inplace update.
When using LFS mode for small-volume devices, IPU will not be used,
and the OPU writing method is actually used, but F2FS_IPU_FORCE can
be read from the ipu_policy node, which is different from the actual
situation. And remount to lfs mode should be disallowed when
f2fs ipu is enabled, let's fix it.

Fixes: 84b89e5d ("f2fs: add auto tuning for small devices")
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>