x86/fpu/xstate: Address kernel-doc warning

Fix kernel-doc warning:

  arch/x86/kernel/fpu/xstate.c:1753: warning: Excess function parameter 'tsk' description in 'fpu_xstate_prctl'

Signed-off-by: Zhu Wang <wangzhu9@huawei.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org