use atomicbitops, not sync/atomic, in refsvfs2

Since refsvfs2 are used in netstack, we should use atomicbitops to avoid
breaking 32-bit builds.

On 64-bit builds there is no performance difference.

PiperOrigin-RevId: 439687980

pkg/sentry/fsimpl/host/BUILD

@@ -29,6 +29,7 @@ go_library(
     visibility = ["//pkg/sentry:internal"],
     deps = [
         "//pkg/abi/linux",
+        "//pkg/atomicbitops",
         "//pkg/context",
         "//pkg/errors/linuxerr",
         "//pkg/fdnotifier",