Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, change the following form:

sizeof(*sgl) + sizeof(sgl->sg[0]) * (MAX_SGL_ENTS + 1)

to :

struct_size(sgl, sg, MAX_SGL_ENTS + 1)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>