block: genhd: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

According to RCU_INIT_POINTER()'s block comment 3.a, it can be used if
"3. The referenced data structure has already been exposed to readers either
at compile time or via rcu_assign_pointer() -and-
a. You have not made -any- reader-visible changes to this structure since
then".

This case fulfills the conditions above because between the rcu_dereference() call and the rcu_assign_pointer() call there is no update of the "part" variable.
Therefore, this patch makes the replacement.

The following Coccinelle semantic patch was used:
@@
identifier v;
@@

v = rcu_dereference(...);

... when != rcu_dereference(...);
    when != v = ...;
    when != (<+...v...+>)++;
    when != \(memcpy\|memset\)(...);

(
- rcu_assign_pointer
+ RCU_INIT_POINTER
(..., v);
|
if(...) {
... when != v = ...;
- rcu_assign_pointer
+ RCU_INIT_POINTER
(..., v);
... when any
}
)

Because there are cases where between a “rcu_dereference()” call and a “rcu_assign_pointer()” call might be updates of the value that interests us, the Coccinelle semantic patch ignores them and replaces with "RCU_INIT_POINTER()" only when the update is not happening.

---
 block/genhd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 3aec057..f7e29d8 100644
@@ -228,7 +228,7 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 		part = rcu_dereference(ptbl->part[i]);
 
 		if (part && sector_in_part(part, sector)) {
-			rcu_assign_pointer(ptbl->last_lookup, part);
+			RCU_INIT_POINTER(ptbl->last_lookup, part);
 			return part;
 		}
 	}

1.9.1