The previous limits were estimated locally in a single step
basead on bound values, however it was not correct since
when given certain scatterlist the function nx_build_sg_lists
was consuming more sg entries than allocated causing a
memory corruption and crashes.

e.g.: in the worst case we could have one sg entry for a single byte.

This patch fixes it modifying the logic of the bound limit
moving it to nx_sg_build_lists and set a correct sg_max limit,
adding a trim function to ensure the bound in sg_list. Also fixing
nx_build_sg_list  NULL and untreated return in case of overflow.
Signed-off-by: Leonidas S. Barbosa <leosilva@linux.vnet.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>