f2fs crypto: do not set encryption policy for non-directory by ioctl

Encryption policy should only be set to an empty directory through ioctl,
This patch add a judgement condition to verify type of the target inode
to avoid incorrectly configuring for non-directory.

Additionally, remove unneeded inline data conversion since regular or symlink
file should not be processed here.

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