head -c is incorrect with FUSE fs with fake stat

[oech3]

With bugfs provided at #9609 (comment) ,

$ gnu-9a3479f1e/head -c 100 /tmp/bugdir/file
foo bar baz⏎
$ head -c 100 /tmp/bugdir/file
foo⏎      

Note that we don't have splice() fast-path yet.

[oech3]

If we simply drop io::copy, it removes it's internal zero-copy optimization too. So #11491 #11620 should be done at 1st.

[oech3]

I considered that this is caused by io::copy()'s splice call, but it seems incorect.

[oech3]

@ChrisDenton I confirmed that replacing std::io::copy with manual read fixed the issue with rustc 1.94.1.
But I cannot reproduce the issue by simple std::io::copy program and FUSE bugfs.

Maybe, Rust should reconsider rust-lang/rust#116451