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: I823edbeddf6cc5086e4d17262d7c497368b1acb7
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@google.com>