smack: Guard smack_ipv6_lock definition within a SMACK_IPV6_PORT_LABELING block

The mutex smack_ipv6_lock is only used with the SMACK_IPV6_PORT_LABELING
block but its definition is outside of the block. This leads to a
defined-but-not-used warning on PREEMPT_RT.

Moving smack_ipv6_lock down to the block where it is used where it used
raises the question why is smk_ipv6_port_list read if nothing is added
to it.
Turns out, only smk_ipv6_port_check() is using it outside of an ifdef
SMACK_IPV6_PORT_LABELING block. However two of three caller invoke
smk_ipv6_port_check() from a ifdef block and only one is using
__is_defined() macro which requires the function and smk_ipv6_port_list
to be around.

Put the lock and list inside an ifdef SMACK_IPV6_PORT_LABELING block to
avoid the warning regarding unused mutex. Extend the ifdef-block to also
cover smk_ipv6_port_check(). Make smack_socket_connect() use ifdef
instead of __is_defined() to avoid complains about missing function.

Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>