tracing/hwlat: Drop the duplicate assignment in start_kthread()

We have set 'current_mask' to '&save_cpumask' in its declaration,
so there is no need to assign again.

Link: https://lkml.kernel.org/r/20200730082318.42584-1-haokexin@gmail.com



Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>