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/9p/client.c

@@ -2070,7 +2070,7 @@ int p9_client_readdir(struct p9_fid *fid, char *data, u32 count, u64 offset)
 	struct kvec kv = {.iov_base = data, .iov_len = count};
 	struct iov_iter to;
 
-	iov_iter_kvec(&to, READ | ITER_KVEC, &kv, 1, count);
+	iov_iter_kvec(&to, READ, &kv, 1, count);
 
 	p9_debug(P9_DEBUG_9P, ">>> TREADDIR fid %d offset %llu count %d\n",
 				fid->fid, (unsigned long long) offset, count);