Description: rds: Fix NULL pointer dereference in __rds_rdma_map
References:
 https://bugzilla.redhat.com/show_bug.cgi?id=1527393
 https://patchwork.kernel.org/patch/10096441/
 https://xorl.wordpress.com/2017/12/18/linux-kernel-rdma-null-pointer-dereference/
Notes:
Bugs:
upstream: released (4.15-rc3) [f3069c6d33f6ae63a1668737bc78aaaa51bff7ca]
4.9-upstream-stable: released (4.9.70) [3259862dd73bfb9d9b7a647ea77cb20ba8b179a4]
3.16-upstream-stable: released (3.16.55) [39961200584fe03d2915886ac49e7ec7a8b5a4ae]
3.2-upstream-stable: released (3.2.100) [cbe131eb2d7bab9b3332094ae279fed7cb170a85]
sid: released (4.14.7-1)
4.9-stretch-security: released (4.9.80-1)
3.16-jessie-security: released (3.16.56-1)
3.2-wheezy-security: released (3.2.101-1)