[CIFS] Cleanup use of CONFIG_CIFS_STATS2 ifdef to make transport routines more readable

Christoph had requested that the stats related code (in
CONFIG_CIFS_STATS2) be moved into helpers to make code flow more
readable.   This patch should help.   For example the following
section from transport.c

                       spin_unlock(&GlobalMid_Lock);
                       atomic_inc(&ses->server->num_waiters);
                       wait_event(ses->server->request_q,
                                  atomic_read(&ses->server->inFlight)
                                    < cifs_max_pending);
                       atomic_dec(&ses->server->num_waiters);
                       spin_lock(&GlobalMid_Lock);

becomes simpler (with the patch below):
                       spin_unlock(&GlobalMid_Lock);
                       cifs_num_waiters_inc(server);
                       wait_event(server->request_q,
                                  atomic_read(&server->inFlight)
                                    < cifs_max_pending);
                       cifs_num_waiters_dec(server);
                       spin_lock(&GlobalMid_Lock);

Reviewed-by: Jeff Layton <jlayton@redhat.com>
CC: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>