Without the barrier_data() inside memzero_explicit(), the compiler may
optimize away the state-clearing if it can tell that the state is not
used afterwards.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>