fuse: use iov_iter based generic splice helpers

The default splice implementation is grossly inefficient and the iter based
ones work just fine, so use those instead.  I've measured an 8x speedup for
splice write (with len = 128k).
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -3156,6 +3156,7 @@ static const struct file_operations fuse_file_operations = {
 	.lock		= fuse_file_lock,
 	.flock		= fuse_file_flock,
 	.splice_read	= generic_file_splice_read,
+	.splice_write	= iter_file_splice_write,
 	.unlocked_ioctl	= fuse_file_ioctl,
 	.compat_ioctl	= fuse_file_compat_ioctl,
 	.poll		= fuse_file_poll,
@@ -3174,11 +3175,12 @@ static const struct file_operations fuse_direct_io_file_operations = {
 	.fsync		= fuse_fsync,
 	.lock		= fuse_file_lock,
 	.flock		= fuse_file_flock,
+	.splice_read	= generic_file_splice_read,
+	.splice_write	= iter_file_splice_write,
 	.unlocked_ioctl	= fuse_file_ioctl,
 	.compat_ioctl	= fuse_file_compat_ioctl,
 	.poll		= fuse_file_poll,
 	.fallocate	= fuse_file_fallocate,
-	/* no splice_read */
 };
 
 static const struct address_space_operations fuse_file_aops  = {