MFC r305389:

With clang 3.9.0, compiling sys/netinet/igmp.c results in the following
warning:

sys/netinet/igmp.c:546:21: error: implicit conversion from 'int' to 'char' changes value from 148 to -108 [-Werror,-Wconstant-conversion]

p->ipopt_list[0] = IPOPT_RA;    /* Router Alert Option */
                 ~ ^~~~~~~~

sys/netinet/ip.h:153:19: note: expanded from macro 'IPOPT_RA'
#define IPOPT_RA 148 /* router alert */

^~~

This is because ipopt_list is an array of char, so IPOPT_RA is wrapped
to a negative value. It would be nice to change ipopt_list to an array
of u_char, but it changes the signature of the public struct ipoption,
so add an explicit cast to suppress the warning.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D7777