kernel: Allow a kernel thread's name to be set in copy_process

This patch allows kernel users to pass in the thread name so it can be
set during creation instead of having to use set_task_comm after the
thread is created.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>