CRED: Restore const to current_cred()

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 <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
cc: Al Viro <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>