fix from_process_at return error in old kernel

[qq906907952]

In the function "FDInfo::from_process_at" it use "rustix::fs::openat" to open file like "/proc/{pid}/fd/x"
which will encounter the 'Os { code: 40, kind: FilesystemLoop, message: "Symbolic link loop" }' error in old version kernel
(test in 2.6.32-696.16.1.el6.x86_64, i guess the some flag not handle correct or not support in old kernel version, espcially SYMLINK_NOFOLLOW)

when i remove SYMLINK_NOFOLLOW flag in openat , "rustix::fs::openat" raise 'Os { code: 6, kind: Uncategorized, message: "No such device or address" }' error

it also encounter 'Os { code: 22, kind: InvalidInput, message: "Invalid argument" }' when get metadata use rustix::fs::statat with flags.

so i remove the "rustix::fs::openat" and remove all AtFlags in "rustix::fs::statat". only use "rustix::fs::readlinkat" and "rustix::fs::statat" to get the fd type and metadata

also remove the unused field "root" in struct FDsIter

it work in old kernel (i only test in 2.6.32-696.16.1.el6.x86_64) and new kernel

[eminence]

Hi, thanks for the bug report, and for the potential fix. I would like to be able to support 2.6 kernels, but getting ahold of one for testing is a bit challenging. I need to do some more research to understand the impact of removing SYMLINK_NOFOLLOW and EMPTY_PATH from the calls to statat