fs: fix FileAllocater type assertion

The type assertion was meant to operate on `f.file`,
not on `n.ops` again.

Fixes xfstests generic/228.

Change-Id: I8f8ca0cead91a512a6d7826dd7e7e5d1887ae627

fs/bridge.go

@@ -811,7 +811,7 @@ func (b *rawBridge) Fallocate(cancel <-chan struct{}, input *fuse.FallocateIn) f
 	if a, ok := n.ops.(NodeAllocater); ok {
 		return errnoToStatus(a.Allocate(&fuse.Context{Caller: input.Caller, Cancel: cancel}, f.file, input.Offset, input.Length, input.Mode))
 	}
-	if a, ok := n.ops.(FileAllocater); ok {
+	if a, ok := f.file.(FileAllocater); ok {
 		return errnoToStatus(a.Allocate(&fuse.Context{Caller: input.Caller, Cancel: cancel}, input.Offset, input.Length, input.Mode))
 	}
 	return fuse.ENOTSUP

posixtest/test.go

@@ -39,6 +39,7 @@ var All = map[string]func(*testing.T, string){
 	"ReadDirPicksUpCreate":       ReadDirPicksUpCreate,
 	"DirectIO":                   DirectIO,
 	"OpenAt":                     OpenAt,
+	"Fallocate":                  Fallocate,
 }
 
 func DirectIO(t *testing.T, mnt string) {
@@ -600,3 +601,23 @@ func OpenAt(t *testing.T, mnt string) {
 		t.Error(err)
 	}
 }
+
+func Fallocate(t *testing.T, mnt string) {
+	rwFile, err := os.OpenFile(mnt+"/file", os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0666)
+	if err != nil {
+		t.Fatalf("OpenFile failed: %v", err)
+	}
+	defer rwFile.Close()
+	err = syscall.Fallocate(int(rwFile.Fd()), 0, 1024, 4096)
+	if err != nil {
+		t.Fatalf("Fallocate failed: %v", err)
+	}
+	fi, err := os.Lstat(mnt + "/file")
+	if err != nil {
+		t.Fatalf("Lstat failed: %v", err)
+	}
+	if fi.Size() < (1024 + 4096) {
+		t.Fatalf("fallocate should have changed file size. Got %d bytes",
+			fi.Size())
+	}
+}