cifs: convert cifs_tcp_ses_lock from a rwlock to a spinlock

cifs_tcp_ses_lock is a rwlock with protects the cifs_tcp_ses_list,
server->smb_ses_list and the ses->tcon_list. It also protects a few
ref counters in server, ses and tcon. In most cases the critical section
doesn't seem to be large, in a few cases where it is slightly large, there
seem to be really no benefit from concurrent access. I briefly considered RCU
mechanism but it appears to me that there is no real need.

Replace it with a spinlock and get rid of the last rwlock in the cifs code.

Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifssmb.c

@@ -593,9 +593,9 @@ CIFSSMBNegotiate(unsigned int xid, struct cifsSesInfo *ses)
 			rc = -EIO;
 			goto neg_err_exit;
 		}
-		read_lock(&cifs_tcp_ses_lock);
+		spin_lock(&cifs_tcp_ses_lock);
 		if (server->srv_count > 1) {
-			read_unlock(&cifs_tcp_ses_lock);
+			spin_unlock(&cifs_tcp_ses_lock);
 			if (memcmp(server->server_GUID,
 				   pSMBr->u.extended_response.
 				   GUID, 16) != 0) {
@@ -605,7 +605,7 @@ CIFSSMBNegotiate(unsigned int xid, struct cifsSesInfo *ses)
 					16);
 			}
 		} else {
-			read_unlock(&cifs_tcp_ses_lock);
+			spin_unlock(&cifs_tcp_ses_lock);
 			memcpy(server->server_GUID,
 			       pSMBr->u.extended_response.GUID, 16);
 		}