proc: include values in /proc/net/netstat

[Haihan-Jiang]

Fixes #13151.

/proc/net/netstat currently emits only the TcpExt field-name row in gVisor. Linux emits netstat rows in name/value pairs, and consumers such as metric parsers expect the value row to have the same number of fields as the header row. Without the value row, parsers report a field-count mismatch.

This change keeps the existing TcpExt fields and emits a matching TcpExt value row populated with zeroes for the stats gVisor does not track yet. That resolves the header/data length mismatch without pretending to implement non-zero counters.

I also added a proc unit test for matching name/value field counts and enabled the existing /proc/net/netstat syscall format check under gVisor.

Validation:

• gofmt -w pkg/sentry/fsimpl/proc/task_net.go pkg/sentry/fsimpl/proc/tasks_sys_test.go
• git diff --check
• attempted bazel test //pkg/sentry/fsimpl/proc:proc_test //test/syscalls/linux:proc_net_test on macOS; local Bazel failed before executing tests due the host toolchain missing C++ standard library headers for external deps.
• attempted SDKROOT=$(xcrun --show-sdk-path) ... bazel test //pkg/sentry/fsimpl/proc:proc_test; local Bazel then failed before executing tests because /usr/bin/aarch64-linux-gnu-gcc is not installed.
• attempted go test ./pkg/sentry/fsimpl/proc; unsupported for this repo checkout because generated proto packages/build-constrained files are provided through Bazel.