cifs: Enable large folio support

Now that cifs is using netfslib for its VM interaction, it only sees I/O in
terms of iov_iter iterators and does not see pages or folios.  This makes
large multipage folios transparent to cifs and so we can turn on multipage
folios on regular files.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org