ext4 crypto: policies may only be set on directories

Thanks to Chao Yu <chao2.yu@samsung.com> for pointing out we were
missing this check.

Change-Id: Ic947578c72d457ae026124e328c981db0bc8c949
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>