btrfs: relocation: Fix KASAN report on create_reloc_tree due to extended reloc tree...

btrfs: relocation: Fix KASAN report on create_reloc_tree due to extended reloc tree lifepsan

[BUG]
When running workload with balance start/cancel, snapshot
creation/deletion and fsstress, we can hit the following KASAN report:

  ==================================================================
  BUG: KASAN: use-after-free in create_reloc_root+0x9f/0x460 [btrfs]
  Read of size 8 at addr ffff8881571741f0 by task btrfs/3539

  CPU: 6 PID: 3539 Comm: btrfs Tainted: G           O      5.5.0-rc1-custom+ #40
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  Call Trace:
   dump_stack+0xc2/0x11a
   print_address_description.constprop.0+0x20/0x210
   __kasan_report.cold+0x1b/0x41
   kasan_report+0x12/0x20
   __asan_load8+0x54/0x90
   create_reloc_root+0x9f/0x460 [btrfs]
   btrfs_reloc_post_snapshot+0xff/0x6c0 [btrfs]
   create_pending_snapshot+0xa9b/0x15f0 [btrfs]
   create_pending_snapshots+0x111/0x140 [btrfs]
   btrfs_commit_transaction+0x7a6/0x1360 [btrfs]
   btrfs_mksubvol+0x915/0x960 [btrfs]
   btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs]
   btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs]
   btrfs_ioctl+0x241b/0x3e60 [btrfs]
   do_vfs_ioctl+0x831/0xb10
   ksys_ioctl+0x67/0x90
   __x64_sys_ioctl+0x43/0x50
   do_syscall_64+0x79/0xe0
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

[CAUSE]
This is another case where root->reloc_root is accessed without checking
if the reloc root is already dead.

[FIX]
Also check DEAD_RELOC_TREE bit before accessing root->reloc_root.

Reported-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots")
Signed-off-by: Qu Wenruo <wqu@suse.com>
(cherry picked from commit 6ededf16b6f30018eed84ea70e5b3cb4a2ad4ebd)