netfs: Provide an iterator-reset function

Provide a function to reset the iterator on a subrequest.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-17-dhowells@redhat.com/

 # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>