kernel: RDMA/core: Make sure "ib_port" is valid when access sysfs node

A NULL pointer dereference flaw was found in the Linux kernel RDMA core's sysfs handling. A local user with access to InfiniBand sysfs nodes can read sysfs attributes while an InfiniBand port is being removed, causing the kernel to dereference a NULL ibport pointer when the port structure has bee...