dm clone: make array 'descs' static

Don't populate the read-only array descs on the stack but instead it
static and add extra const. Also makes the object code smaller by 66
bytes:

Before:
   text    data     bss     dec     hex filename
  42382   11140     512   54034    d312 ./drivers/md/dm-clone-target.o

After:
   text    data     bss     dec     hex filename
  42220   11236     512   53968    d2d0 ./drivers/md/dm-clone-target.o

(gcc version 11.2.0)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-clone-target.c

@@ -161,7 +161,7 @@ static const char *clone_device_name(struct clone *clone)
 
 static void __set_clone_mode(struct clone *clone, enum clone_metadata_mode new_mode)
 {
-	const char *descs[] = {
+	static const char * const descs[] = {
 		"read-write",
 		"read-only",
 		"fail"