vrf: Handle CONFIG_SYSCTL not set

Randy reported compile failure when CONFIG_SYSCTL is not set/enabled:

ERROR: modpost: "sysctl_vals" [drivers/net/vrf.ko] undefined!

Fix by splitting out the sysctl init and cleanup into helpers that
can be set to do nothing when CONFIG_SYSCTL is disabled. In addition,
move vrf_strict_mode and vrf_strict_mode_change to above
vrf_shared_table_handler (code move only) and wrap all of it
in the ifdef CONFIG_SYSCTL.

Update the strict mode tests to check for the existence of the
/proc/sys entry.

Fixes: 33306f1aaf ("vrf: add sysctl parameter for strict mode")
Cc: Andrea Mayer <andrea.mayer@uniroma2.it>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David Ahern <dsahern@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/vrf_strict_mode_test.sh

@@ -379,6 +379,12 @@ if [ ! -x "$(command -v ip)" ]; then
 	exit 0
 fi
 
+modprobe vrf &>/dev/null
+if [ ! -e /proc/sys/net/vrf/strict_mode ]; then
+	echo "SKIP: vrf sysctl does not exist"
+	exit 0
+fi
+
 cleanup &> /dev/null
 
 setup