modules/swcore: bugfix, make sure that IB does not request force_free before start page is written to the LL

If frame is to be dropped while only one page (start page) was used, then the
start page is written two times to the Linked List. First time, with the inter
page as the next_page; second time with next_page set to 0, and EOF high. That's
because inter page was not yet used so only start_page should be force-freed.
It happened once in a while that IB was requesting force_free of a received
frame before start_page with EOF was written to the linked list. Therefore
freeing module was making a mess by trying to force free the start_page, then
the inter page (which was not used in fact) and then trying to read next inter
page (which was not there...).
With this commit I try to make sure that start page is always written to the LL
before force-free request goes from IB to the freeing module.