btrfs: avoid NULL pointer dereference if no valid extent tree

[BUG]
Syzbot reported a crash with the following call trace:

 BTRFS info (device loop0): scrub: started on devid 1
 BUG: kernel NULL pointer dereference, address: 0000000000000208
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 106e70067 P4D 106e70067 PUD 107143067 PMD 0
 Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
 CPU: 1 UID: 0 PID: 689 Comm: repro Kdump: loaded Tainted: G           O       6.13.0-rc4-custom+ torvalds#206
 Tainted: [O]=OOT_MODULE
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
 RIP: 0010:find_first_extent_item+0x26/0x1f0 [btrfs]
 Call Trace:
  <TASK>
  scrub_find_fill_first_stripe+0x13d/0x3b0 [btrfs]
  scrub_simple_mirror+0x175/0x260 [btrfs]
  scrub_stripe+0x5d4/0x6c0 [btrfs]
  scrub_chunk+0xbb/0x170 [btrfs]
  scrub_enumerate_chunks+0x2f4/0x5f0 [btrfs]
  btrfs_scrub_dev+0x240/0x600 [btrfs]
  btrfs_ioctl+0x1dc8/0x2fa0 [btrfs]
  ? do_sys_openat2+0xa5/0xf0
  __x64_sys_ioctl+0x97/0xc0
  do_syscall_64+0x4f/0x120
  entry_SYSCALL_64_after_hwframe+0x76/0x7e
  </TASK>

[CAUSE]
The reproducer is using a corrupted image where extent tree root is
corrupted, thus forcing to use "rescue=all,ro" mount option to mount the
image.

Then it triggered a scrub, but since scrub relies on extent tree to find
where the data/metadata extents are, scrub_find_fill_first_stripe()
relies on an non-empty extent root.

But unfortunately scrub_find_fill_first_stripe() doesn't really expect
an NULL pointer for extent root, it use extent_root to grab fs_info and
triggered a NULL pointer dereference.

[FIX]
Add an extra check for a valid extent root at the beginning of
scrub_find_fill_first_stripe().

The new error path is introduced by 42437a6 ("btrfs: introduce
mount option rescue=ignorebadroots"), but that's pretty old, and later
commit b979547 ("btrfs: scrub: introduce helper to find and fill
sector info for a scrub_stripe") changed how we do scrub.

So for kernels older than 6.6, the fix will need manual backport.

Reported-by: [email protected]
Link: https://lore.kernel.org/linux-btrfs/[email protected]/
Fixes: 42437a6 ("btrfs: introduce mount option rescue=ignorebadroots")
Signed-off-by: Qu Wenruo <[email protected]>

fs/btrfs/scrub.c

@@ -1475,6 +1475,10 @@ static int scrub_find_fill_first_stripe(struct btrfs_block_group *bg,
 	u64 extent_gen;
 	int ret;
 
+	if (unlikely(!extent_root)) {
+		btrfs_err(fs_info, "no valid extent root for scrub");
+		return -EUCLEAN;
+	}
 	memset(stripe->sectors, 0, sizeof(struct scrub_sector_verification) *
 				   stripe->nr_sectors);
 	scrub_stripe_reset_bitmaps(stripe);