x86/ldt: Initialize the context lock for init_mm

The mutex mm->context->lock for init_mm is not initialized for init_mm.
This wasn't a problem because it remained unused. This changed however
since commit
	4fc19708b1 ("x86/alternatives: Initialize temporary mm for patching")

Initialize the mutex for init_mm.

Fixes: 4fc19708b1 ("x86/alternatives: Initialize temporary mm for patching")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Nadav Amit <namit@vmware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20190701173354.2pe62hhliok2afea@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/mmu.h

@@ -59,6 +59,7 @@ typedef struct {
 #define INIT_MM_CONTEXT(mm)						\
 	.context = {							\
 		.ctx_id = 1,						\
+		.lock = __MUTEX_INITIALIZER(mm.context.lock),		\
 	}
 
 void leave_mm(int cpu);