staging: fwserial: Switch to kfree_rcu() API

Instead of invoking a synchronize_rcu() to free a pointer after a grace
period, we can directly make use of a new API that does the same but in
a more efficient way.

Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Link: https://lore.kernel.org/r/20220920135145.1387-1-shangxiaojing@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>