fuse: add fast path for fuse_range_is_writeback

fuse: add fast path for fuse_range_is_writeback

In some cases, the fi->writepages may be empty. And there is no need
to check fi->writepages with spin_lock, which may have an impact on
performance due to lock contention. For example, in scenarios where
multiple readers read the same file without any writers, or where
the page cache is not enabled.

Also remove the outdated comment since commit 6b2fb79963fb ("fuse:
optimize writepages search") has optimize the situation by replacing
list with rb-tree.

Signed-off-by: yangyun <yangyun50@huawei.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>