staging: lustre: libcfs: check mask returned by cpumask_of_node

cpumask_of_node can return NULL if NUMA node is unavailable,
in this case cfs_node_to_cpumask will try to copy from NULL
and cause kernel panic.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5751
Reviewed-on: http://review.whamcloud.com/13207


Reviewed-by: Li Wei <wei.g.li@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>