ext: boilerplate code.

Renamed ext4 to ext since we are targeting ext(2/3/4).
Removed fs.go since we are targeting VFS2.
Added ext.go with filesystem struct.

PiperOrigin-RevId: 257689775

pkg/sentry/fs/ext/BUILD

@@ -0,0 +1,11 @@
+package(licenses = ["notice"])
+
+load("//tools/go_stateify:defs.bzl", "go_library")
+
+go_library(
+    name = "ext",
+    srcs = ["ext.go"],
+    importpath = "gvisor.dev/gvisor/pkg/sentry/fs/ext",
+    visibility = ["//pkg/sentry:internal"],
+    deps = ["//pkg/sentry/fs/ext/disklayout"],
+)

pkg/sentry/fs/ext/disklayout/BUILD

@@ -21,7 +21,8 @@ go_library(
         "superblock_old.go",
         "test_utils.go",
     ],
-    importpath = "gvisor.dev/gvisor/pkg/sentry/fs/ext4/disklayout",
+    importpath = "gvisor.dev/gvisor/pkg/sentry/fs/ext/disklayout",
+    visibility = ["//pkg/sentry:internal"],
     deps = [
         "//pkg/abi/linux",
         "//pkg/binary",