df: issues when dealing with nested mounts

[cre4ture]

I create this issue from a finding from pullrequest #5821.

When running tests of uutils df on my own android phone (not on AVD),
and after fixing the issue with the integer overflow addressed in #6071,
they are still failing in test test_df::test_type_option_with_file:

---- test_df::test_type_option_with_file stdout ----
run: /data/data/com.termux/files/home/rust/coreutils/target/debug/coreutils df --output=fstype .
run: /data/data/com.termux/files/home/rust/coreutils/target/debug/coreutils df -t f2fs .
run: /data/data/com.termux/files/home/rust/coreutils/target/debug/coreutils df -t nonexisting .
run: /data/data/com.termux/files/home/rust/coreutils/target/debug/coreutils df --output=fstype
run: /data/data/com.termux/files/home/rust/coreutils/target/debug/coreutils df -t erofs .
thread 'test_df::test_type_option_with_file' panicked at tests/by-util/test_df.rs:292:14:
Command was expected to fail.
stdout = Filesystem       1K-blocks   Used Available Use% Mounted on
/dev/block/dm-15    595792 595792         0 100% /

 stderr = 
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_df::test_type_option_with_file

test result: FAILED. 48 passed; 1 failed; 0 ignored; 0 measured; 2946 filtered out; finished in 0.97s

for easier debugging and comparison (termux uses toybox df) I tried to reproduce the issue on my ubuntu.
There I tried to setup a similar setup as I have it on the phone - two nested filesystem mounts with different fstypes.
As my /home - which is a nested mount - has the same fstype as the root, I can't use it for the test.

So I simply mounted a ISO image I recently downloaded and did the tests manually there:

GNU output:

uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ df -T .
Filesystem     Type    1K-blocks   Used Available Use% Mounted on
/dev/loop20    iso9660    617966 617966         0 100% /media/uli/OpenBSD_amd64   7.4 Install CD
uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ df -T -t iso9660 .
Filesystem     Type    1K-blocks   Used Available Use% Mounted on
/dev/loop20    iso9660    617966 617966         0 100% /media/uli/OpenBSD_amd64   7.4 Install CD
uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ df -T -t ext4 .
df: no file systems processed

uutils output:

uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ ~/dev_rust/coreutils/target/debug/coreutils df -T .
Filesystem     Type 1K-blocks     Used Available Use% Mounted on
/dev/nvme0n1p7 ext4 100904268 34242964  61489496  36% /
uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ ~/dev_rust/coreutils/target/debug/coreutils df -T -t iso9660 .
df: no file systems processed
uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ ~/dev_rust/coreutils/target/debug/coreutils df -T -t ext4 .
Filesystem     Type 1K-blocks     Used Available Use% Mounted on
/dev/nvme0n1p7 ext4 100904268 34242964  61489496  36% /

So, even worse then on android, uutils wrongly ignores the nested mount of ISO image and reports type ext4 of root mount.