proc "seq files": switch to ->read_iter

Implement ->read_iter for all proc "seq files" so that splice works on
them.

Suggested-by: Linus Torvalds <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/generic.c

···
 static const struct proc_ops proc_seq_ops = {
 	/* not permanent -- can call into arbitrary seq_operations */
 	.proc_open	= proc_seq_open,
-	.proc_read	= seq_read,
+	.proc_read_iter	= seq_read_iter,
 	.proc_lseek	= seq_lseek,
 	.proc_release	= proc_seq_release,
 };