SMB3: Fix length checking of SMB3.11 negotiate request

The length checking for SMB3.11 negotiate request includes
"negotiate contexts" which caused a buffer validation problem
and a confusing warning message on SMB3.11 mount e.g.:

     SMB2 server sent bad RFC1001 len 236 not 170

Fix the length checking for SMB3.11 negotiate to account for
the new negotiate context so that we don't log a warning on
SMB3.11 mount by default but do log warnings if lengths returned
by the server are incorrect.

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>

fs/cifs/smb2pdu.h

@@ -263,6 +263,13 @@ struct smb2_negotiate_req {
 #define SMB2_NT_FIND			0x00100000
 #define SMB2_LARGE_FILES		0x00200000
 
+struct smb2_neg_context {
+	__le16	ContextType;
+	__le16	DataLength;
+	__le32	Reserved;
+	/* Followed by array of data */
+} __packed;
+
 #define SMB311_SALT_SIZE			32
 /* Hash Algorithm Types */
 #define SMB2_PREAUTH_INTEGRITY_SHA512	cpu_to_le16(0x0001)