ucounts: Proper error handling in set_cred_ucounts

commit 34dc2fd6 upstream.

Instead of leaking the ucounts in new if alloc_ucounts fails, store
the result of alloc_ucounts into a temporary variable, which is later
assigned to new->ucounts.

Cc: stable@vger.kernel.org
Fixes: 905ae01c

 ("Add a reference to ucounts for each cred")
Link: https://lkml.kernel.org/r/87pms2s0v8.fsf_-_@disp2133
Tested-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Alexey Gladkov <legion@kernel.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>