cache: tolerate short writes in print_slot

sendfile() can return after a short read/write, so we may need to call
it more than once. As suggested in the manual page, we fall back to
read/write if sendfile fails with EINVAL or ENOSYS.

On the read/write path, use write_in_full which deals with short writes.

Signed-off-by: Hristo Venev <hristo@venev.name>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>