IB/core: type promotion bug in rdma_rw_init_one_mr()

"nents" is an unsigned int, so if ib_map_mr_sg() returns a negative
error code then it's type promoted to a high unsigned int which is
treated as success.

Fixes: a060b562 ("IB/core: generic RDMA READ/WRITE API")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>