vrf: Update to register_net_sysctl_sz

Move from register_net_sysctl to register_net_sysctl_sz and pass the
ARRAY_SIZE of the ctl_table array that was used to create the table
variable. We need to move to the new function in preparation for when we
change SIZE_MAX to ARRAY_SIZE() in the register_net_sysctl macro.
Failing to do so would erroneously allow ARRAY_SIZE() to be called on a
pointer. The actual change from SIZE_MAX to ARRAY_SIZE will take place
in subsequent commits.

Signed-off-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>