smb3.1.1: set gcm256 when requested

update smb encryption code to set 32 byte key length and to
set gcm256 when requested on mount.

Signed-off-by: Steve French <stfrench@microsoft.com>
diff --git a/fs/cifs/smb2pdu.h b/fs/cifs/smb2pdu.h
index 05b010e..851c6cd 100644
--- a/fs/cifs/smb2pdu.h
+++ b/fs/cifs/smb2pdu.h
@@ -352,6 +352,7 @@ struct smb2_preauth_neg_context {
 /* Encryption Algorithms Ciphers */
 #define SMB2_ENCRYPTION_AES128_CCM	cpu_to_le16(0x0001)
 #define SMB2_ENCRYPTION_AES128_GCM	cpu_to_le16(0x0002)
+/* we currently do not request AES256_CCM since presumably GCM faster */
 #define SMB2_ENCRYPTION_AES256_CCM      cpu_to_le16(0x0003)
 #define SMB2_ENCRYPTION_AES256_GCM      cpu_to_le16(0x0004)