Btrfs: check for empty bitmap list in setup_cluster_bitmaps

Dave Jones found a warning from kasan in setup_cluster_bitmaps()

==================================================================
BUG: KASAN: stack-out-of-bounds in setup_cluster_bitmap+0xc4/0x5a0 at
addr ffff88039bef6828
Read of size 8 by task nfsd/1009
page:ffffea000e6fbd80 count:0 mapcount:0 mapping:          (null)
index:0x0
flags: 0x8000000000000000()
page dumped because: kasan: bad access detected
CPU: 1 PID: 1009 Comm: nfsd Tainted: G        W
4.4.0-rc3-backup-debug+ #1
 ffff880065647b50 000000006bb712c2 ffff88039bef6640 ffffffffa680a43e
 0000004559c00000 ffff88039bef66c8 ffffffffa62638d1 ffffffffa61121c0
 ffff8803a5769de8 0000000000000296 ffff8803a5769df0 0000000000046280
Call Trace:
 [<ffffffffa680a43e>] dump_stack+0x4b/0x6d
 [<ffffffffa62638d1>] kasan_report_error+0x501/0x520
 [<ffffffffa61121c0>] ? debug_show_all_locks+0x1e0/0x1e0
 [<ffffffffa6263948>] kasan_report+0x58/0x60
 [<ffffffffa6814b00>] ? rb_last+0x10/0x40
 [<ffffffffa66f8af4>] ? setup_cluster_bitmap+0xc4/0x5a0
 [<ffffffffa6262ead>] __asan_load8+0x5d/0x70
 [<ffffffffa66f8af4>] setup_cluster_bitmap+0xc4/0x5a0
 [<ffffffffa66f675a>] ? setup_cluster_no_bitmap+0x6a/0x400
 [<ffffffffa66fcd16>] btrfs_find_space_cluster+0x4b6/0x640
 [<ffffffffa66fc860>] ? btrfs_alloc_from_cluster+0x4e0/0x4e0
 [<ffffffffa66fc36e>] ? btrfs_return_cluster_to_free_space+0x9e/0xb0
 [<ffffffffa702dc37>] ? _raw_spin_unlock+0x27/0x40
 [<ffffffffa666a1a1>] find_free_extent+0xba1/0x1520

Andrey noticed this was because we were doing list_first_entry on a list
that might be empty.  Rework the tests a bit so we don't do that.
Signed-off-by: Chris Mason <clm@fb.com>
Reprorted-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Reported-by: Dave Jones <dsj@fb.com>

fs/btrfs/free-space-cache.c

@@ -2972,7 +2972,7 @@ setup_cluster_bitmap(struct btrfs_block_group_cache *block_group,
 		     u64 cont1_bytes, u64 min_bytes)
 {
 	struct btrfs_free_space_ctl *ctl = block_group->free_space_ctl;
-	struct btrfs_free_space *entry;
+	struct btrfs_free_space *entry = NULL;
 	int ret = -ENOSPC;
 	u64 bitmap_offset = offset_to_bitmap(ctl, offset);
 
@@ -2983,8 +2983,10 @@ setup_cluster_bitmap(struct btrfs_block_group_cache *block_group,
 	 * The bitmap that covers offset won't be in the list unless offset
 	 * is just its start offset.
 	 */
+	if (!list_empty(bitmaps))
 		entry = list_first_entry(bitmaps, struct btrfs_free_space, list);
-	if (entry->offset != bitmap_offset) {
+
+	if (!entry || entry->offset != bitmap_offset) {
 		entry = tree_search_offset(ctl, bitmap_offset, 1, 0);
 		if (entry && list_empty(&entry->list))
 			list_add(&entry->list, bitmaps);