ocfs2: fix panic in failed foilio allocation

In the Linux kernel, the following vulnerability has been resolved: ocfs2: fix panic in failed foilio allocation commit 7e119cff9d0a ("ocfs2: convert w_pages to w_folios") and commit 9a5e08652dc4b ("ocfs2: use an array of folios instead of an array of pages") save -ENOMEM in the folio array upon allocation failure and call the folio array free code. The folio array free code expects either valid folio pointers or NULL. Finding the -ENOMEM will result in a panic. Fix by NULLing the error folio entry.