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>

fs/dlm/lowcomms.c

@@ -674,7 +674,7 @@ static int receive_from_sock(struct connection *con)
 		nvec = 2;
 	}
 	len = iov[0].iov_len + iov[1].iov_len;
-	iov_iter_kvec(&msg.msg_iter, READ | ITER_KVEC, iov, nvec, len);
+	iov_iter_kvec(&msg.msg_iter, READ, iov, nvec, len);
 
 	r = ret = sock_recvmsg(con->sock, &msg, MSG_DONTWAIT | MSG_NOSIGNAL);
 	if (ret <= 0)