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: 33306f1a

 ("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>