proc "single files": switch to ->read_iter

Implement ->read_iter for all proc "single files" so that more bionic
tests cases can pass when they call splice() on other fun files like
/proc/version
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/proc/generic.c

@@ -621,7 +621,7 @@ static int proc_single_open(struct inode *inode, struct file *file)
 static const struct proc_ops proc_single_ops = {
 	/* not permanent -- can call into arbitrary ->single_show */
 	.proc_open	= proc_single_open,
-	.proc_read	= seq_read,
+	.proc_read_iter = seq_read_iter,
 	.proc_lseek	= seq_lseek,
 	.proc_release	= single_release,
 };