RDS: Heap OOB write in rds_message_alloc_sgs()

RDS: Heap OOB write in rds_message_alloc_sgs()

Orabug: 27934066
CVE: CVE-2018-5332

When args->nr_local is 0, nr_pages gets also 0 due some size
calculation via rds_rm_size(), which is later used to allocate
pages for DMA, this bug produces a heap Out-Of-Bound write access
to a specific memory region.

Signed-off-by: Mohamed Ghannam <simo.ghannam@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit c095508770aebf1b9218e77026e48345d719b17c)
Signed-off-by: Brian Maly <brian.maly@oracle.com>