Eric reported an issue where mounting -o recovery with a fuzzed fs
resulted in a kernel panic.  This is because we tried to free the tree
node, except it was an error from the read.  Fix this by properly
resetting the tree_root->node == NULL in this case.  The panic was the
following

  BTRFS warning (device loop0): failed to read tree root
  BUG: kernel NULL pointer dereference, address: 000000000000001f
  RIP: 0010:free_extent_buffer+0xe/0x90 [btrfs]
  Call Trace:
   free_root_extent_buffers.part.0+0x11/0x30 [btrfs]
   free_root_pointers+0x1a/0xa2 [btrfs]
   open_ctree+0x1776/0x18a5 [btrfs]
   btrfs_mount_root.cold+0x13/0xfa [btrfs]
   ? selinux_fs_context_parse_param+0x37/0x80
   legacy_get_tree+0x27/0x40
   vfs_get_tree+0x25/0xb0
   fc_mount+0xe/0x30
   vfs_kern_mount.part.0+0x71/0x90
   btrfs_mount+0x147/0x3e0 [btrfs]
   ? cred_has_capability+0x7c/0x120
   ? legacy_get_tree+0x27/0x40
   legacy_get_tree+0x27/0x40
   vfs_get_tree+0x25/0xb0
   do_mount+0x735/0xa40
   __x64_sys_mount+0x8e/0xd0
   do_syscall_64+0x4d/0x90
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

Nik says: this is problematic only if we fail on the last iteration of
the loop as this results in init_tree_roots returning err value with
tree_root->node = -ERR. Subsequently the caller does: fail_tree_roots
which calls free_root_pointers on the bogus value.
Reported-by: Eric Sandeen <sandeen@redhat.com>
Fixes: b8522a1e ("btrfs: Factor out tree roots initialization during mount")
CC: stable@vger.kernel.org # 5.5+
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ add details how the pointer gets dereferenced ]
Signed-off-by: David Sterba <dsterba@suse.com>