Filipe reported the following lockdep splat

  ======================================================
  WARNING: possible circular locking dependency detected
  5.10.0-rc2-btrfs-next-71 #1 Not tainted
  ------------------------------------------------------
  find/324157 is trying to acquire lock:
  ffff8ebc48d293a0 (btrfs-tree-01#2/3){++++}-{3:3}, at: __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]

  but task is already holding lock:
  ffff8eb9932c5088 (btrfs-tree-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #1 (btrfs-tree-00){++++}-{3:3}:
	 lock_acquire+0xd8/0x490
	 down_write_nested+0x44/0x120
	 __btrfs_tree_lock+0x27/0x120 [btrfs]
	 btrfs_search_slot+0x2a3/0xc50 [btrfs]
	 btrfs_insert_empty_items+0x58/0xa0 [btrfs]
	 insert_with_overflow+0x44/0x110 [btrfs]
	 btrfs_insert_xattr_item+0xb8/0x1d0 [btrfs]
	 btrfs_setxattr+0xd6/0x4c0 [btrfs]
	 btrfs_setxattr_trans+0x68/0x100 [btrfs]
	 __vfs_setxattr+0x66/0x80
	 __vfs_setxattr_noperm+0x70/0x200
	 vfs_setxattr+0x6b/0x120
	 setxattr+0x125/0x240
	 path_setxattr+0xba/0xd0
	 __x64_sys_setxattr+0x27/0x30
	 do_syscall_64+0x33/0x80
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  -> #0 (btrfs-tree-01#2/3){++++}-{3:3}:
	 check_prev_add+0x91/0xc60
	 __lock_acquire+0x1689/0x3130
	 lock_acquire+0xd8/0x490
	 down_read_nested+0x45/0x220
	 __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]
	 btrfs_next_old_leaf+0x27d/0x580 [btrfs]
	 btrfs_real_readdir+0x1e3/0x4b0 [btrfs]
	 iterate_dir+0x170/0x1c0
	 __x64_sys_getdents64+0x83/0x140
	 do_syscall_64+0x33/0x80
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  other info that might help us debug this:

   Possible unsafe locking scenario:

	 CPU0                    CPU1
	 ----                    ----
    lock(btrfs-tree-00);
				 lock(btrfs-tree-01#2/3);
				 lock(btrfs-tree-00);
    lock(btrfs-tree-01#2/3);

   *** DEADLOCK ***

  5 locks held by find/324157:
   #0: ffff8ebc502c6e00 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0x4d/0x60
   #1: ffff8eb97f689980 (&type->i_mutex_dir_key#10){++++}-{3:3}, at: iterate_dir+0x52/0x1c0
   #2: ffff8ebaec00ca58 (btrfs-tree-02#2){++++}-{3:3}, at: __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]
   #3: ffff8eb98f986f78 (btrfs-tree-01#2){++++}-{3:3}, at: __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]
   #4: ffff8eb9932c5088 (btrfs-tree-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]

  stack backtrace:
  CPU: 2 PID: 324157 Comm: find Not tainted 5.10.0-rc2-btrfs-next-71 #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
  Call Trace:
   dump_stack+0x8d/0xb5
   check_noncircular+0xff/0x110
   ? mark_lock.part.0+0x468/0xe90
   check_prev_add+0x91/0xc60
   __lock_acquire+0x1689/0x3130
   ? kvm_clock_read+0x14/0x30
   ? kvm_sched_clock_read+0x5/0x10
   lock_acquire+0xd8/0x490
   ? __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]
   down_read_nested+0x45/0x220
   ? __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]
   __btrfs_tree_read_lock+0x32/0x1a0 [btrfs]
   btrfs_next_old_leaf+0x27d/0x580 [btrfs]
   btrfs_real_readdir+0x1e3/0x4b0 [btrfs]
   iterate_dir+0x170/0x1c0
   __x64_sys_getdents64+0x83/0x140
   ? filldir+0x1d0/0x1d0
   do_syscall_64+0x33/0x80
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

This happens because btrfs_next_old_leaf searches down to our current
key, and then walks up the path until we can move to the next slot, and
then reads back down the path so we get the next leaf.

However it doesn't unlock any lower levels until it replaces them with
the new extent buffer.  This is technically fine, but of course causes
lockdep to complain, because we could be holding locks on lower levels
while locking upper levels.

Fix this by dropping all nodes below the level that we use as our new
starting point before we start reading back down the path.  This also
allows us to drop the nested/recursive locking magic, because we're no
longer locking two nodes at the same level anymore.
Reported-by: Filipe Manana <fdmanana@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>