cifs: Fix TCP_Server_Info::credits to be signed

Fix TCP_Server_Info::credits to be signed, just as echo_credits and
oplock_credits are.  This also fixes what ought to get at least a
compilation warning if not an outright error in *get_credits_field() as a
pointer to the unsigned server->credits field is passed back as a pointer
to a signed int.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cifs@vger.kernel.org
Cc: stable@vger.kernel.org
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Acked-by: Pavel Shilovskiy <pshilovskiy@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>