CIFS: Clarify SMB1 code for UnixSetPathInfo

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 doing SetPathInfo (setattr) when using the Unix
extensions.  This doesn't change the address but makes it
slightly clearer.

Addresses-Coverity: 711528 ("Out of bounds read")
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifssmb.c

@@ -6064,9 +6064,8 @@ setPermsRetry:
 	param_offset = offsetof(struct smb_com_transaction2_spi_req,
 				InformationLevel) - 4;
 	offset = param_offset + params;
-	data_offset =
-	    (FILE_UNIX_BASIC_INFO *) ((char *) &pSMB->hdr.Protocol +
-				      offset);
+	/* SMB offsets are from the beginning of SMB which is 4 bytes in, after RFC1001 field */
+	data_offset = (FILE_UNIX_BASIC_INFO *)((char *) pSMB + offset + 4);
 	memset(data_offset, 0, count);
 	pSMB->DataOffset = cpu_to_le16(offset);
 	pSMB->ParameterOffset = cpu_to_le16(param_offset);