sockfilt.c: Fixed compilation warnings

sockfilt.c:288: warning: conversion to 'DWORD' from 'size_t' may alter
                its value
sockfilt.c:291: warning: conversion to 'DWORD' from 'size_t' may alter
                its value
sockfilt.c:323: warning: conversion to 'DWORD' from 'size_t' may alter
                its value
sockfilt.c:326: warning: conversion to 'DWORD' from 'size_t' may alter
                its value