RDMA: Fix allocation failure on pointer pd

The null check on an allocation failure on pd is currently checking
if pd is non-null rather than null. Fix this by adding the missing !
operator.

Fixes: 21a428a0

 ("RDMA: Handle PD allocations by IB/core")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>