crypto: api - Add crypto_tfm_ctx_dma

This patch adds the helpers crypto_tfm_ctx_aligned and
crypto_tfm_ctx_dma.  The first aligns the tfm context to the
value cra_alignmask.  The second sets the alignment according
to dma_cache_get_alignment();

This patch also moves crypto_tfm_ctx into algapi.h.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>