Fix coverity warning

Coverity reports a warning for referencing the beginning of the
SMB2/SMB3 frame using the ProtocolId field as an array. Although
it works the same either way, this patch should quiet the warning
and might be a little clearer.

Reported by Coverity (CID 741269)

Signed-off-by: Steve French <smfrench@gmail.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Acked-by: Sachin Prabhu <sprabhu@redhat.com>
Reviewed-by: Jeff Layton <jlayton@poochiereds.net>

fs/cifs/smb2misc.c

@@ -322,7 +322,7 @@ smb2_get_data_area_len(int *off, int *len, struct smb2_hdr *hdr)
 
 	/* return pointer to beginning of data area, ie offset from SMB start */
 	if ((*off != 0) && (*len != 0))
-		return hdr->ProtocolId + *off;
+		return (char *)(&hdr->ProtocolId[0]) + *off;
 	else
 		return NULL;
 }