Syzbot reports a warning in close_fs_devices that happens because
fs_devices->rw_devices is not 0 after calling btrfs_close_one_device
on each device.

This happens when a writeable device is removed in
__btrfs_free_extra_devids, but the rw device count is not decremented
accordingly. So when close_fs_devices is called, the removed device is
still counted and we get an off by 1 error.

Here is one call trace that was observed:
   btrfs_mount_root():
     btrfs_scan_one_device():
       device_list_add();   <---------------- device added
     btrfs_open_devices():
       open_fs_devices():
         btrfs_open_one_device();   <-------- rw device count ++
     btrfs_fill_super():
       open_ctree():
         btrfs_free_extra_devids():
	  __btrfs_free_extra_devids();  <--- device removed
	  fail_tree_roots:
	    btrfs_close_devices():
	      close_fs_devices();   <------- rw device count off by 1

Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device")
Reported-by: syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
Tested-by: syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
Signed-off-by: Desmond Cheong Zhi Xi <redacted>
---
  fs/btrfs/volumes.c | 1 +
  1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 807502cd6510..916c25371658 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c

@@ -1078,6 +1078,7 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
  		if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
  			list_del_init(&device->dev_alloc_list);
  			clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
+			fs_devices->rw_devices--;
  		}
  		list_del_init(&device->dev_list);
  		fs_devices->num_devices--;