ucounts: In set_cred_ucounts assume new->ucounts is non-NULL

Any cred that is destined for use by commit_creds must have a non-NULL
cred->ucounts field.  Only curing credential construction is a NULL
cred->ucounts valid.  Only abort_creds, put_cred, and put_cred_rcu
needs to deal with a cred with a NULL ucount.  As set_cred_ucounts is
non of those case don't confuse people by handling something that can
not happen.

Link: https://lkml.kernel.org/r/871r4irzds.fsf_-_@disp2133Tested-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Alexey Gladkov <legion@kernel.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

kernel/cred.c

@@ -676,15 +676,14 @@ int set_cred_ucounts(struct cred *new)
 	 * This optimization is needed because alloc_ucounts() uses locks
 	 * for table lookups.
 	 */
-	if (old_ucounts && old_ucounts->ns == new->user_ns && uid_eq(old_ucounts->uid, new->euid))
+	if (old_ucounts->ns == new->user_ns && uid_eq(old_ucounts->uid, new->euid))
 		return 0;
 
 	if (!(new_ucounts = alloc_ucounts(new->user_ns, new->euid)))
 		return -EAGAIN;
 
 	new->ucounts = new_ucounts;
-	if (old_ucounts)
-		put_ucounts(old_ucounts);
+	put_ucounts(old_ucounts);
 
 	return 0;
 }