bug#55910: cp error

[pk]

Package: coreutils

- Boot arch linux live cd 2022.06. It has coreutils 9.1-1.
- mkfs.btrfs /dev/vda
- mount /dev/vda /mnt
- cp -ax / /mnt
cp: cannot create directory '/mnt/': No such file or directory

If relevant: strace says that newfstatat and mkdirat have been given
"" as pathname and return ENOENT.

Instead, the exact same steps work on arch linux 2022.03, which has
coreutils 9.0-2. Note that -x should prevent trying to copy /mnt under
itself; on 2022.03, there is a proper error message for cp -a / /mnt
without -x.