[iov_iter] new primitives - copy_from_iter_full() and friends

copy_from_iter_full(), copy_from_iter_full_nocache() and
csum_and_copy_from_iter_full() - counterparts of copy_from_iter()
et.al., advancing iterator only in case of successful full copy
and returning whether it had been successful or not.

Convert some obvious users.  *NOTE* - do not blindly assume that
something is a good candidate for those unless you are sure that
not advancing iov_iter in failure case is the right thing in
this case.  Anything that does short read/short write kind of
stuff (or is in a loop, etc.) is unlikely to be a good one.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/net/tipc/msg.c b/net/tipc/msg.c
index 17201aa..a22be50 100644
--- a/net/tipc/msg.c
+++ b/net/tipc/msg.c
@@ -268,7 +268,7 @@
 		__skb_queue_tail(list, skb);
 		skb_copy_to_linear_data(skb, mhdr, mhsz);
 		pktpos = skb->data + mhsz;
-		if (copy_from_iter(pktpos, dsz, &m->msg_iter) == dsz)
+		if (copy_from_iter_full(pktpos, dsz, &m->msg_iter))
 			return dsz;
 		rc = -EFAULT;
 		goto error;
@@ -299,7 +299,7 @@
 		if (drem < pktrem)
 			pktrem = drem;
 
-		if (copy_from_iter(pktpos, pktrem, &m->msg_iter) != pktrem) {
+		if (!copy_from_iter_full(pktpos, pktrem, &m->msg_iter)) {
 			rc = -EFAULT;
 			goto error;
 		}