x86/pkru: Provide pkru_write_default()

Provide a simple and trivial helper which just writes the PKRU default
value without trying to fiddle with the task's xsave buffer.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121455.513729794@linutronix.de

arch/x86/include/asm/pkru.h

@@ -60,4 +60,12 @@ static inline void write_pkru(u32 pkru)
 	fpregs_unlock();
 }
 
+static inline void pkru_write_default(void)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
+		return;
+
+	wrpkru(pkru_get_init_value());
+}
+
 #endif