CIFS: Clarify SMB1 code for rename open file

Coverity also complains about the way we calculate the offset
(starting from the address of a 4 byte array within the
header structure rather than from the beginning of the struct
plus 4 bytes) for SMB1 RenameOpenFile. This changeset
doesn't change the address but makes it slightly clearer.

Addresses-Coverity: 711521 ("Out of bounds write")
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifssmb.c

@@ -2767,7 +2767,8 @@ int CIFSSMBRenameOpenFile(const unsigned int xid, struct cifs_tcon *pTcon,
 	param_offset = offsetof(struct smb_com_transaction2_sfi_req, Fid) - 4;
 	offset = param_offset + params;
 
-	data_offset = (char *) (&pSMB->hdr.Protocol) + offset;
+	/* SMB offsets are from the beginning of SMB which is 4 bytes in, after RFC1001 field */
+	data_offset = (char *)(pSMB) + offset + 4;
 	rename_info = (struct set_file_rename *) data_offset;
 	pSMB->MaxParameterCount = cpu_to_le16(2);
 	pSMB->MaxDataCount = cpu_to_le16(1000); /* BB find max SMB from sess */