os: os.Root: use openat2 with RESOLVE_BENEATH on Linux

[neild]

Linux's openat2 syscall with the RESOLVE_BENEATH flag opens a file within some root directory. We should use it when available.

openat2 is only supported since Linux 5.6, so we'll still need a fallback on older versions.

[gabyhelp]

Related Issues

• os: os.Root: use O_RESOLVE_BENEATH on FreeBSD #73078

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