ocfs2: reset folio to NULL when get folio fails

The reproducer uses FAULT_INJECTION to make memory allocation fail, which
causes __filemap_get_folio() to fail, when initializing w_folios[i] in
ocfs2_grab_folios_for_write(), it only returns an error code and the value
of w_folios[i] is the error code, which causes
ocfs2_unlock_and_free_folios() to recycle the invalid w_folios[i] when
releasing folios.

Link: https://lkml.kernel.org/r/20250616013140.3602219-1-lizhi.xu@windriver.com
Reported-by: syzbot+c2ea94ae47cd7e3881ec@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c2ea94ae47cd7e3881ec
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>