cifs: Transition from ->readpages() to ->readahead()

Transition the cifs filesystem from using the old ->readpages() method to
using the new ->readahead() method.

For the moment, this removes any invocation of fscache to read data from
the local cache, leaving that to another patch.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <smfrench@gmail.com>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: linux-cachefs@redhat.com
Reviewed-by: Rohith Surabattula <rohiths@microsoft.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>