CRED: Restore const to current_cred()

Commit 3295514841c2 ("fix rcu annotations noise in cred.h") accidentally
dropped the const of current->cred inside current_cred() by the
insertion of a cast to deal with an RCU annotation loss warning from
sparce.

Use an appropriate RCU wrapper instead so as not to lose the const.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cred.h

@@ -269,7 +269,7 @@ static inline void put_cred(const struct cred *_cred)
  * since nobody else can modify it.
  */
 #define current_cred() \
-	(*(__force struct cred **)&current->cred)
+	rcu_dereference_protected(current->cred, 1)
 
 /**
  * __task_cred - Access a task's objective credentials