target/iscsi: Replace conn->login_ip with login_sockaddr

Very similar to how it went with local_sockaddr.

It was embedded in iscsi_login_stats so some changes there, and we needed
to copy in a sockaddr_storage comparison function. Hopefully the kernel
will get a standard one soon, our implementation makes the 3rd.

isert_set_conn_info() became much smaller.

IPV6_ADDRESS_SPACE define goes away, had to modify a call to in6_pton(),
can just use -1 since we are sure string is null-terminated.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>