arch_set_user_pkey_access only works on the current task_struct

[Jiashuo Liang]

Hi,

I am learning the kernel implementation of the x86 PKU feature. I find the
arch_set_user_pkey_access function in arch/x86/kernel/fpu/xstate.c does not
use its first parameter. So it is perhaps a bug?

The arch_set_user_pkey_access function is supposed to set the PKRU register
for the task_struct specified by its first parameter tsk. But it is only
implemented for the current task_struct.

Fortunately, it has been called only with current task_struct in the kernel
code, so it appears to be okay. However, it can introduce bugs in the
future because people may expect it working on other task_struct.

This commit seems to be related: b79daf8589921 ("x86/mm/pkeys: Fix compact
mode by removing protection keys' XSAVE buffer manipulation").

Thank you!
liangjs