powerpc/mm/hash: Check for non-kernel address in get_kernel_vsid()

get_kernel_vsid() has a very stern comment saying that it's only valid
for kernel addresses, but there's nothing in the code to enforce that.

Rather than hoping our callers are well behaved, add a check and return
a VSID of 0 (invalid).
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/mmu-hash.h

@@ -682,6 +682,9 @@ static inline unsigned long get_kernel_vsid(unsigned long ea, int ssize)
 {
 	unsigned long context;
 
+	if (!is_kernel_addr(ea))
+		return 0;
+
 	/*
 	 * For kernel space, we use context ids 1-4 to map the address space as
 	 * below: