ANDROID: GKI: fscrypt: add ABI padding to struct fscrypt_operations

'struct fscrypt_operations' shouldn't really be part of the KMI, as
there's no reason for loadable modules to use it.  However, due to the
way MODVERSIONS calculates symbol CRCs by recursively dereferencing
structures, changes to 'struct fscrypt_operations' affect the CRCs of
KMI functions exported from certain core kernel files such as
fs/dcache.c.  That brings it in-scope for the KMI freeze.

Therefore, add some reserved fields to this struct for LTS updates.

Bug: 151154716
Change-Id: Ic3bf66c93a9be167a0a5b257bd55e2719d99a1b4
Signed-off-by: Eric Biggers <ebiggers@google.com>