When makeFieldBookmark() method is called with
ODF_COMMENTRANGE it will ignore the added field name and
generate an own one. We have to set the name of the
SwPostItField to this generated name so these two names will match.

Note: lTagBkmk is only an identifier but not the exported name
of the fieldmark so we don't need to stick to use it as
a name.

Change-Id: I499abdcce1be0563c308bcf9f5c0a959a07f858b