x86/cpu: Write the default PKRU value when enabling PKE

In preparation of making the PKRU management more independent from XSTATES,
write the default PKRU value into the hardware right after enabling PKRU in
CR4. This ensures that switch_to() and copy_thread() have the correct
setting for init task and the per CPU idle threads right away.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121455.622983906@linutronix.de