Bug 207975

# Error description
Trying to delete subvolume which is set as default, not allowed.
States reason to dmesg but not to terminal.

# Request:
Output to terminal as well, the reason why can't delete subvolume. In example: because subvolume is set as default on deletion intent.

# Environment
uname -a
Linux damian 5.4.0-33-generic #37-Ubuntu SMP Thu May 21 12:53:59 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

btrfs --version
btrfs-progs v5.4.1 

sudo btrfs fi show
Label: none  uuid: c41a0bb3-2e82-314c-456a-cffda8400b36
	Total devices 1 FS bytes used 11.95GiB
	devid    1 size 180.00GiB used 16.07GiB path /dev/nbd3p1

btrfs fi df /drive/
Data, single: total=14.01GiB, used=11.23GiB
System, DUP: total=32.00MiB, used=16.00KiB
Metadata, DUP: total=1.00GiB, used=89.67MiB
GlobalReserve, single: total=12.84MiB, used=0.00B

dmesg | grep BTRFS
[  221.769923] BTRFS: device fsid c41a0bb3-2e82-314c-456a-cffda8400b36 devid 1 transid 1039 /dev/nbd3p1
[  253.000241] BTRFS info (device nbd3p1): disk space caching is enabled
[  253.000244] BTRFS info (device nbd3p1): has skinny extents
[  253.093369] BTRFS info (device nbd3p1): enabling ssd optimizations
[ 1359.791080] BTRFS info (device nbd3p1): disk space caching is enabled
[ 1359.791081] BTRFS info (device nbd3p1): has skinny extents
[ 1359.870093] BTRFS info (device nbd3p1): enabling ssd optimizations

# Current behavior -> error replication:
sudo mount -t btrfs -o subvolid=5 /dev/nbd3p1 /drive/

sudo btrfs subvolume create /drive/newvol
Create subvolume '/drive/newvol'

sudo btrfs subvolume list /drive
ID 464 gen 1190 top level 5 path newvol

sudo btrfs subvolume set-default 464 /drive/

sudo btrfs subvolume get-default  /drive/
ID 464 gen 1190 top level 5 path newvol

# On deletion intent
sudo btrfs subvolume delete /drive/newvol/
Delete subvolume (no-commit): '/drive/newvol'
ERROR: Could not destroy subvolume/snapshot: Operation not permitted

dmesg | grep BTRFS
[ 5160.908633] BTRFS error (device nbd3p1): deleting default subvolume 464 is not allowed

# Effective deletion
sudo btrfs subvolume set-default 5 /drive/

sudo btrfs subvolume delete /drive/newvol/
Delete subvolume (no-commit): '/drive/newvol'