splice: remove permission hook from iter_file_splice_write()

All the callers of ->splice_write(), (e.g. do_splice_direct() and
do_splice()) already check rw_verify_area() for the entire range
and perform all the other checks that are in vfs_write_iter().

Instead of creating another tiny helper for special caller, just
open-code it.

This is needed for fanotify "pre content" events.
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231122122715.2561213-6-amir73il@gmail.comSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/splice.c

@@ -673,10 +673,13 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 		.u.file = out,
 	};
 	int nbufs = pipe->max_usage;
-	struct bio_vec *array = kcalloc(nbufs, sizeof(struct bio_vec),
-					GFP_KERNEL);
+	struct bio_vec *array;
 	ssize_t ret;
 
+	if (!out->f_op->write_iter)
+		return -EINVAL;
+
+	array = kcalloc(nbufs, sizeof(struct bio_vec), GFP_KERNEL);
 	if (unlikely(!array))
 		return -ENOMEM;
 
@@ -684,6 +687,7 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 
 	splice_from_pipe_begin(&sd);
 	while (sd.total_len) {
+		struct kiocb kiocb;
 		struct iov_iter from;
 		unsigned int head, tail, mask;
 		size_t left;
@@ -733,7 +737,10 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 		}
 
 		iov_iter_bvec(&from, ITER_SOURCE, array, n, sd.total_len - left);
-		ret = vfs_iter_write(out, &from, &sd.pos, 0);
+		init_sync_kiocb(&kiocb, out);
+		kiocb.ki_pos = sd.pos;
+		ret = call_write_iter(out, &kiocb, &from);
+		sd.pos = kiocb.ki_pos;
 		if (ret <= 0)
 			break;