Double swapon on a device causes a crash:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
PGD 1dc0b067 PUD 1dc09067 PMD 0
Oops: 0000 [#1] SMP
last sysfs file:
CPU 1
Modules linked in:
Pid: 562, comm: swapon Tainted: G        W  2.6.32-rc5-mm1_64 #867
RIP: 0010:[<ffffffff810af160>]  [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
...

It is due to swap_info_struct->first_swap_extent.list not being
initialized. ->next is NULL in such a situation and
destroy_swap_extents fails to iterate over the list with the BUG
above.

Introduced by swap_info-include-first_swap_extent.patch. Revert the
INIT_LIST_HEAD move.
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swapfile.c

@@ -1365,7 +1365,6 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
 	if (start_page == 0) {
 		se = &sis->first_swap_extent;
 		sis->curr_swap_extent = se;
-		INIT_LIST_HEAD(&se->list);
 		se->start_page = 0;
 		se->nr_pages = nr_pages;
 		se->start_block = start_block;
@@ -1818,6 +1817,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		kfree(p);
 		goto out;
 	}
+	INIT_LIST_HEAD(&p->first_swap_extent.list);
 	if (type >= nr_swapfiles) {
 		p->type = type;
 		swap_info[type] = p;