powerpc/kexec_file: Fix error code when trying to load kdump kernel

kexec_file_load() on powerpc doesn't support kdump kernels yet, so it
returns -ENOTSUPP in that case.

I've recently learned that this errno is internal to the kernel and
isn't supposed to be exposed to userspace. Therefore, change to
-EOPNOTSUPP which is defined in an uapi header.

This does indeed make kexec-tools happier. Before the patch, on
ppc64le:

  # ~bauermann/src/kexec-tools/build/sbin/kexec -s -p /boot/vmlinuz
  kexec_file_load failed: Unknown error 524

After the patch:

  # ~bauermann/src/kexec-tools/build/sbin/kexec -s -p /boot/vmlinuz
  kexec_file_load failed: Operation not supported

Fixes: a0458284 ("powerpc: Add support code for kexec_file_load()")
Cc: stable@vger.kernel.org # v4.10+
Reported-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Reviewed-by: Simon Horman <horms@verge.net.au>
Reviewed-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/machine_kexec_file_64.c

@@ -43,7 +43,7 @@ int arch_kexec_kernel_image_probe(struct kimage *image, void *buf,
 
 	/* We don't support crash kernels yet. */
 	if (image->type == KEXEC_TYPE_CRASH)
-		return -ENOTSUPP;
+		return -EOPNOTSUPP;
 
 	for (i = 0; i < ARRAY_SIZE(kexec_file_loaders); i++) {
 		fops = kexec_file_loaders[i];