x86/fpu: Cleanup arch_set_user_pkey_access()

The function does a sanity check with a WARN_ON_ONCE() but happily proceeds
when the pkey argument is out of range.

Clean it up.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121453.635764326@linutronix.de