Remove kernfs dev id cache

Kernfs checking was previously done for every directory scanned, but the
new parallel scanning code only performs the check when the dev id is
different from parent, which isn't nearly as common.
(In fact, in typical scenarios this only ever happens once per dev id,
rendering the cache completely useless. But even people will 10k bind
mounts are unlikely to notice a performance impact)

src/scan.zig

@@ -12,14 +12,7 @@ const c_statfs = @cImport(@cInclude("sys/vfs.h"));
 
 
 // This function only works on Linux
-fn isKernfs(dir: std.fs.Dir, dev: u64) bool {
+fn isKernfs(dir: std.fs.Dir) bool {
-    const state = struct {
-        var lock: std.Thread.Mutex = .{};
-        var cache: std.AutoHashMap(u64,bool) = std.AutoHashMap(u64,bool).init(main.allocator);
-    };
-    state.lock.lock();
-    defer state.lock.unlock();
-    if (state.cache.get(dev)) |e| return e;
     var buf: c_statfs.struct_statfs = undefined;
     if (c_statfs.fstatfs(dir.fd, &buf) != 0) return false; // silently ignoring errors isn't too nice.
     const iskern = switch (util.castTruncate(u32, buf.f_type)) {
@@ -39,7 +32,6 @@ fn isKernfs(dir: std.fs.Dir, dev: u64) bool {
         => true,
         else => false,
     };
-    state.cache.put(dev, iskern) catch {};
     return iskern;
 }
 
@@ -247,7 +239,7 @@ const Thread = struct {
         if (@import("builtin").os.tag == .linux
             and main.config.exclude_kernfs
             and stat.dev != dir.dev
-            and isKernfs(edir, stat.dev)
+            and isKernfs(edir)
         ) {
             edir.close();
             dir.sink.addSpecial(t.sink, name, .kernfs);