RDMA/cma: Avoid triggering undefined behavior

According to the C standard the behavior of computations with
integer operands is as follows:
* A computation involving unsigned operands can never overflow,
  because a result that cannot be represented by the resulting
  unsigned integer type is reduced modulo the number that is one
  greater than the largest value that can be represented by the
  resulting type.
* The behavior for signed integer underflow and overflow is
  undefined.

Hence only use unsigned integers when checking for integer
overflow.

This patch is what I came up with after having analyzed the
following smatch warnings:

drivers/infiniband/core/cma.c:3448: cma_resolve_ib_udp() warn: signed overflow undefined. 'offset + conn_param->private_data_len < conn_param->private_data_len'
drivers/infiniband/core/cma.c:3505: cma_connect_ib() warn: signed overflow undefined. 'offset + conn_param->private_data_len < conn_param->private_data_len'

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>