mds: convert fscrypt flag to two opaque fields

mds: convert fscrypt flag to two opaque fields

A flag isn't sufficient as we can't reasonably use an xattr to store the
context. Switch the fscrypt fields to two vectors of opaque bytes, one
governed by AUTH caps and the other by FILE.

Also remove the special handling for encryption.ctx xattr, since we
won't be using that going forward anyway.

Signed-off-by: Jeff Layton <jlayton@redhat.com>