ocfs2: fix panic in failed foilio allocation

commit 7e119cf ("ocfs2: convert w_pages to w_folios") and commit
9a5e086 ("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.

Link: https://lkml.kernel.org/r/c879a52b-835c-4fa0-902b-8b2e9196dcbd@oracle.com
Fixes: 7e119cf ("ocfs2: convert w_pages to w_folios")
Fixes: 9a5e086 ("ocfs2: use an array of folios instead of an array of pages")
Signed-off-by: Mark Tinguely <mark.tinguely@oracle.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/ocfs2/alloc.c

@@ -6918,6 +6918,7 @@ static int ocfs2_grab_folios(struct inode *inode, loff_t start, loff_t end,
 		if (IS_ERR(folios[numfolios])) {
 			ret = PTR_ERR(folios[numfolios]);
 			mlog_errno(ret);
+			folios[numfolios] = NULL;
 			goto out;
 		}