rdma_cm: Detect recursive calls to rdma_seterrno()

rdma_cm: Detect recursive calls to rdma_seterrno()

If rdma_seterrno() is called in a recursive fashion, the
result is that errno ends up set to -1.  We can avoid this
by only setting errno if the return value is > 0.

Problem reported by hookenz@gmail.com.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>