Converting RAID1 to single fails if RAID1 is missing device

[Jeff Siddall <news@siddall.name>]

After a device failed on RAID1 filesystem, an attempt to convert the 
online filesystem from RAID1 to single failed.  This isn't an uncommon 
use case if the failed device isn't readily replaceable.

The command ran was:

btrfs balance start -f -sconvert=single -mconvert=single 
-dconvert=single /mountpoint

and the kernel logs were:

kernel: BTRFS info (device nvme0n1p3): balance: start -dconvert=single 
-mconvert=dup -sconvert=dup
kernel: BTRFS info (device nvme0n1p3): relocating block group 
1222049267712 flags data|raid1
kernel: BTRFS warning (device nvme0n1p3): chunk 1223123009536 missing 1 
devices, max tolerance is 0 for writable mount
kernel: BTRFS: error (device nvme0n1p3) in write_all_supers:4370: 
errno=-5 IO failure (errors while submitting device barriers.)
kernel: BTRFS info (device nvme0n1p3: state E): forced readonly
kernel: BTRFS warning (device nvme0n1p3: state E): Skipping commit of 
aborted transaction.
kernel: BTRFS: error (device nvme0n1p3: state EA) in 
cleanup_transaction:1992: errno=-5 IO failure
kernel: BTRFS info (device nvme0n1p3: state EA): balance: ended with 
status: -5

Either it should be made possible to convert a RAID1 device with a 
missing device to a single device filesystem without errors, or the 
command should return a message stating that it is not supported to 
convert RAID1 array with missing devices to a single.  Having the 
process fail and then going forced readonly is a significant failure on 
an otherwise working system.