Fix getsockopt() for IP_OPTIONS/IP_RETOPTS.

r336616 copies inp->inp_options using the m_dup() function.
However, this function expects an mbuf packet header at the beginning,
which is not true in this case.
Therefore, use m_copym() instead of m_dup().

This issue was found by syzkaller.
Reviewed by:		mmacy@
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D18753

sys/netinet/ip_output.c

@@ -1263,7 +1263,8 @@ ip_ctloutput(struct socket *so, struct sockopt *sopt)
 			if (inp->inp_options) {
 				struct mbuf *options;
 
-				options = m_dup(inp->inp_options, M_NOWAIT);
+				options = m_copym(inp->inp_options, 0,
+				    M_COPYALL, M_NOWAIT);
 				INP_RUNLOCK(inp);
 				if (options != NULL) {
 					error = sooptcopyout(sopt,