rename mroute_extract_addr_ipv4 to mroute_extract_addr_ip

rename mroute_extract_addr_ipv4 to mroute_extract_addr_ip

mroute_extract_addr_ipv4() is able to extract an IPv4 as well as an
IPv6. Remove the "v4" suffix from its name to make this behaviour more
explicit.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170724143559.11503-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15129.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 3b38c43b8d7aa22b3df12029ff43e0414891e48c)