iov_iter: Separate type from direction and use accessor functions

In the iov_iter struct, separate the iterator type from the iterator
direction and use accessor functions to access them in most places.

Convert a bunch of places to use switch-statements to access them rather
then chains of bitwise-AND statements.  This makes it easier to add further
iterator types.  Also, this can be more efficient as to implement a switch
of small contiguous integers, the compiler can use ~50% fewer compare
instructions than it has to use bitwise-and instructions.

Further, cease passing the iterator type into the iterator setup function.
The iterator function can set that itself.  Only the direction is required.

Signed-off-by: David Howells <dhowells@redhat.com>

net/socket.c

@@ -635,7 +635,7 @@ EXPORT_SYMBOL(sock_sendmsg);
 int kernel_sendmsg(struct socket *sock, struct msghdr *msg,
 		   struct kvec *vec, size_t num, size_t size)
 {
-	iov_iter_kvec(&msg->msg_iter, WRITE | ITER_KVEC, vec, num, size);
+	iov_iter_kvec(&msg->msg_iter, WRITE, vec, num, size);
 	return sock_sendmsg(sock, msg);
 }
 EXPORT_SYMBOL(kernel_sendmsg);
@@ -648,7 +648,7 @@ int kernel_sendmsg_locked(struct sock *sk, struct msghdr *msg,
 	if (!sock->ops->sendmsg_locked)
 		return sock_no_sendmsg_locked(sk, msg, size);
 
-	iov_iter_kvec(&msg->msg_iter, WRITE | ITER_KVEC, vec, num, size);
+	iov_iter_kvec(&msg->msg_iter, WRITE, vec, num, size);
 
 	return sock->ops->sendmsg_locked(sk, msg, msg_data_left(msg));
 }
@@ -823,7 +823,7 @@ int kernel_recvmsg(struct socket *sock, struct msghdr *msg,
 	mm_segment_t oldfs = get_fs();
 	int result;
 
-	iov_iter_kvec(&msg->msg_iter, READ | ITER_KVEC, vec, num, size);
+	iov_iter_kvec(&msg->msg_iter, READ, vec, num, size);
 	set_fs(KERNEL_DS);
 	result = sock_recvmsg(sock, msg, flags);
 	set_fs(oldfs);