plugins/pppoatm: Restructure code to avoid possibility of integer overflow

plugins/pppoatm: Restructure code to avoid possibility of integer overflow

This avoids the theoretical possibility of integer overflow in
adding a constant before dividing in order to get the effect of
rounding up.  Instead we divide and add 1 if the original value modulo
the divisor is non-zero.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>