fs/netfs: remove `netfs_io_request.ractl`

Since this field is only used by netfs_prepare_read_iterator() when
called by netfs_readahead(), we can simply pass it as parameter.  This
shrinks the struct from 576 to 568 bytes.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/20250519134813.2975312-8-dhowells@redhat.com


cc: Paulo Alcantara <pc@manguebit.com>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>