RDMA/cma: fix null-ptr-deref Read in cma_cleanup

RDMA/cma: fix null-ptr-deref Read in cma_cleanup

In cma_init, if cma_configfs_init fails, need to free the
previously memory and return fail, otherwise will trigger
null-ptr-deref Read in cma_cleanup.

cma_cleanup
  cma_configfs_exit
    configfs_unregister_subsystem

Fixes: 045959db65c6 ("IB/cma: Add configfs for rdma_cm")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Link: https://lore.kernel.org/r/1566188859-103051-1-git-send-email-zhengbin13@huawei.com
Signed-off-by: Doug Ledford <dledford@redhat.com>