netfs: Allow the netfs to make the io (sub)request alloc larger

Allow the network filesystem to specify extra space to be allocated on the
end of the io (sub)request.  This allows cifs, for example, to use this
space rather than allocating its own cifs_readdata struct.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org