vrf: Handle CONFIG_SYSCTL not set

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: 33306f1aaf82 ("vrf: add sysctl parameter for strict mode")
Cc: Andrea Mayer <[email protected]>
Reported-by: Randy Dunlap <[email protected]>
Signed-off-by: David Ahern <[email protected]>
Acked-by: Randy Dunlap <[email protected]> # build-tested
Signed-off-by: David S. Miller <[email protected]>