update require_kernel_version to handle "_" in version string
update require_kernel_version to handle "_" in version string

test code breaks on fedora 33
```
$ cargo test
failures:

---- sys::test_socket::recvfrom::udp_offload::gro stdout ----
thread 'sys::test_socket::recvfrom::udp_offload::gro' panicked at 'called `Result::unwrap()` on an `Err` value: ParseError("Extra junk after valid version: _64")', test/sys/test_socket.rs:292:13
```

this is due underscore in release string( arch/x86_64), which is not supported by semver.
```
$ uname -r
5.8.14-300.fc33.x86_64
```

Replace the underscore with hypen to provide a consistent sematic.

asomers/nix,carllerche/nix-rust,geofft/nix-rust,nix-rust/nix,nix-rust/nix,geofft/nix-rust,asomers/nix,carllerche/nix-rust,carllerche/nix-rust,geofft/nix-rust