fscrypt: Remove __packed from fscrypt_policy

This commit removes __packed from fscrypt_policy as it does not contain
any implicit padding and does not refer to an on-disk structure. Even
though this is a change to a UAPI file, no users will be broken as the
structure doesn't change.

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