Move PreallocateFile to fs package

internal/fs/preallocate_linux.go

@@ -0,0 +1,16 @@
+package fs
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func PreallocateFile(wr *os.File, size int64) error {
+	if size <= 0 {
+		return nil
+	}
+	// int fallocate(int fd, int mode, off_t offset, off_t len)
+	// use mode = 0 to also change the file size
+	return unix.Fallocate(int(wr.Fd()), 0, 0, size)
+}