slirp: Use DIV_ROUND_UP

Replace (((n) + (d) - 1) /(d)) by DIV_ROUND_UP(n,d).

This patch is the result of coccinelle script
scripts/coccinelle/round.cocci

CC: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>