RDS: RDMA: fix the ib_map_mr_sg_zbva() argument

[ Upstream commit 3e56c2f8 ]

Fixes warning: Using plain integer as NULL pointer

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>