Add new entry to the RDMA-CM configfs that allows users
to select default TOS for RDMA-CM QPs.

This is useful for users that want to control the TOS for legacy
applications without changing their code.

Application that sets the TOS explicitly using the rdma_set_option
API will continue to work as expected, meaning overriding the configfs
value.

CC: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>