rds: Fix warning.

[ Upstream commit d84e7bc0595a7e146ad0ddb80b240cea77825245 ]

>> net/rds/send.c:1109:42: warning: Using plain integer as NULL pointer

Fixes: ea010070d0a7 ("net/rds: fix warn in rds_message_alloc_sgs")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>