In some cases the zero-length hw_desc array at the end of
ablkcipher_edesc struct requires for 4B of tail padding.

Due to tail padding and the way pointers to S/G table and IV
are computed:
	edesc->sec4_sg = (void *)edesc + sizeof(struct ablkcipher_edesc) +
			 desc_bytes;
	iv = (u8 *)edesc->hw_desc + desc_bytes + sec4_sg_bytes;
first 4 bytes of IV are overwritten by S/G table.

Update computation of pointer to S/G table to rely on offset of hw_desc
member and not on sizeof() operator.

Cc: <stable@vger.kernel.org> # 4.13+
Fixes: 115957bb ("crypto: caam - fix IV DMA mapping and updating")
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>