net: fix typo in arp clean up

The clean up patch missed an &, so we end up passing an int rather than
a pointer to the sprintf function.

arp.c: In function 'ArpReceive':
arp.c:197: warning: format '%p' expects type 'void *', but argument 3 has type 'int'

Signed-off-by: Mike Frysinger <vapier@gentoo.org>