[BUG]
With fuzzed image and MIXED_GROUPS super flag, we can hit the following
BUG_ON():

  kernel BUG at fs/btrfs/delayed-ref.c:491!
  invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
  CPU: 0 PID: 1849 Comm: sync Tainted: G           O      5.2.0-custom #27
  RIP: 0010:update_existing_head_ref.cold+0x44/0x46 [btrfs]
  Call Trace:
   add_delayed_ref_head+0x20c/0x2d0 [btrfs]
   btrfs_add_delayed_tree_ref+0x1fc/0x490 [btrfs]
   btrfs_free_tree_block+0x123/0x380 [btrfs]
   __btrfs_cow_block+0x435/0x500 [btrfs]
   btrfs_cow_block+0x110/0x240 [btrfs]
   btrfs_search_slot+0x230/0xa00 [btrfs]
   ? __lock_acquire+0x105e/0x1e20
   btrfs_insert_empty_items+0x67/0xc0 [btrfs]
   alloc_reserved_file_extent+0x9e/0x340 [btrfs]
   __btrfs_run_delayed_refs+0x78e/0x1240 [btrfs]
   ? kvm_clock_read+0x18/0x30
   ? __sched_clock_gtod_offset+0x21/0x50
   btrfs_run_delayed_refs.part.0+0x4e/0x180 [btrfs]
   btrfs_run_delayed_refs+0x23/0x30 [btrfs]
   btrfs_commit_transaction+0x53/0x9f0 [btrfs]
   btrfs_sync_fs+0x7c/0x1c0 [btrfs]
   ? __ia32_sys_fdatasync+0x20/0x20
   sync_fs_one_sb+0x23/0x30
   iterate_supers+0x95/0x100
   ksys_sync+0x62/0xb0
   __ia32_sys_sync+0xe/0x20
   do_syscall_64+0x65/0x240
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

[CAUSE]
This situation is caused by several factors:
- Fuzzed image
  The extent tree of this fs missed one backref for extent tree root.
  So we can allocated space from that slot.

- MIXED_BG feature
  Super block has MIXED_BG flag.

- No mixed block groups exists
  All block groups are just regular ones.

This makes data space_info->block_groups[] contains metadata block
groups.  And when we reserve space for data, we can use space in
metadata block group.

Then we hit the following file operations:

- fallocate
  We need to allocate data extents.
  find_free_extent() choose to use the metadata block to allocate space
  from, and choose the space of extent tree root, since its backref is
  missing.

  This generate one delayed ref head with is_data = 1.

- extent tree update
  We need to update extent tree at run_delayed_ref time.

  This generate one delayed ref head with is_data = 0, for the same
  bytenr of old extent tree root.

Then we trigger the BUG_ON().

[FIX]
The quick fix here is to check block_group->flags before using it.

The problem can only happen for MIXED_GROUPS fs. Regular filesystems
won't have space_info with DATA|METADATA flag, and no way to hit the
bug.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=203255Reported-by: Jungyeon Yoon <jungyeon.yoon@gmail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>