When enabling qgroups we walk the tree_root and then add a qgroup item
for every root that we have.  This creates a lock dependency on the
tree_root and qgroup_root, which results in the following lockdep splat
(with tree locks using rwsem), eg. in tests btrfs/017 or btrfs/022:

  ======================================================
  WARNING: possible circular locking dependency detected
  5.9.0-default+ #1299 Not tainted
  ------------------------------------------------------
  btrfs/24552 is trying to acquire lock:
  ffff9142dfc5f630 (btrfs-quota-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]

  but task is already holding lock:
  ffff9142dfc5d0b0 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #1 (btrfs-root-00){++++}-{3:3}:
	 __lock_acquire+0x3fb/0x730
	 lock_acquire.part.0+0x6a/0x130
	 down_read_nested+0x46/0x130
	 __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]
	 __btrfs_read_lock_root_node+0x3a/0x50 [btrfs]
	 btrfs_search_slot_get_root+0x11d/0x290 [btrfs]
	 btrfs_search_slot+0xc3/0x9f0 [btrfs]
	 btrfs_insert_item+0x6e/0x140 [btrfs]
	 btrfs_create_tree+0x1cb/0x240 [btrfs]
	 btrfs_quota_enable+0xcd/0x790 [btrfs]
	 btrfs_ioctl_quota_ctl+0xc9/0xe0 [btrfs]
	 __x64_sys_ioctl+0x83/0xa0
	 do_syscall_64+0x2d/0x70
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  -> #0 (btrfs-quota-00){++++}-{3:3}:
	 check_prev_add+0x91/0xc30
	 validate_chain+0x491/0x750
	 __lock_acquire+0x3fb/0x730
	 lock_acquire.part.0+0x6a/0x130
	 down_read_nested+0x46/0x130
	 __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]
	 __btrfs_read_lock_root_node+0x3a/0x50 [btrfs]
	 btrfs_search_slot_get_root+0x11d/0x290 [btrfs]
	 btrfs_search_slot+0xc3/0x9f0 [btrfs]
	 btrfs_insert_empty_items+0x58/0xa0 [btrfs]
	 add_qgroup_item.part.0+0x72/0x210 [btrfs]
	 btrfs_quota_enable+0x3bb/0x790 [btrfs]
	 btrfs_ioctl_quota_ctl+0xc9/0xe0 [btrfs]
	 __x64_sys_ioctl+0x83/0xa0
	 do_syscall_64+0x2d/0x70
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  other info that might help us debug this:

   Possible unsafe locking scenario:

	 CPU0                    CPU1
	 ----                    ----
    lock(btrfs-root-00);
				 lock(btrfs-quota-00);
				 lock(btrfs-root-00);
    lock(btrfs-quota-00);

   *** DEADLOCK ***

  5 locks held by btrfs/24552:
   #0: ffff9142df431478 (sb_writers#10){.+.+}-{0:0}, at: mnt_want_write_file+0x22/0xa0
   #1: ffff9142f9b10cc0 (&fs_info->subvol_sem){++++}-{3:3}, at: btrfs_ioctl_quota_ctl+0x7b/0xe0 [btrfs]
   #2: ffff9142f9b11a08 (&fs_info->qgroup_ioctl_lock){+.+.}-{3:3}, at: btrfs_quota_enable+0x3b/0x790 [btrfs]
   #3: ffff9142df431698 (sb_internal#2){.+.+}-{0:0}, at: start_transaction+0x406/0x510 [btrfs]
   #4: ffff9142dfc5d0b0 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]

  stack backtrace:
  CPU: 1 PID: 24552 Comm: btrfs Not tainted 5.9.0-default+ #1299
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
  Call Trace:
   dump_stack+0x77/0x97
   check_noncircular+0xf3/0x110
   check_prev_add+0x91/0xc30
   validate_chain+0x491/0x750
   __lock_acquire+0x3fb/0x730
   lock_acquire.part.0+0x6a/0x130
   ? __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]
   ? lock_acquire+0xc4/0x140
   ? __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]
   down_read_nested+0x46/0x130
   ? __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]
   __btrfs_tree_read_lock+0x35/0x1c0 [btrfs]
   ? btrfs_root_node+0xd9/0x200 [btrfs]
   __btrfs_read_lock_root_node+0x3a/0x50 [btrfs]
   btrfs_search_slot_get_root+0x11d/0x290 [btrfs]
   btrfs_search_slot+0xc3/0x9f0 [btrfs]
   btrfs_insert_empty_items+0x58/0xa0 [btrfs]
   add_qgroup_item.part.0+0x72/0x210 [btrfs]
   btrfs_quota_enable+0x3bb/0x790 [btrfs]
   btrfs_ioctl_quota_ctl+0xc9/0xe0 [btrfs]
   __x64_sys_ioctl+0x83/0xa0
   do_syscall_64+0x2d/0x70
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fix this by dropping the path whenever we find a root item, add the
qgroup item, and then re-lookup the root item we found and continue
processing roots.
Reported-by: David Sterba <dsterba@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>