sysctl: Add ctl_table_size to ctl_table_header

The new ctl_table_size element will hold the size of the ctl_table
arrays contained in the ctl_table_header. This value should eventually
be passed by the callers to the sysctl register infrastructure. And
while this commit introduces the variable, it does not set nor use it
because that requires case by case considerations for each caller.

It provides two important things: (1) A place to put the
result of the ctl_table array calculation when it gets introduced for
each caller. And (2) the size that will be used as the additional
stopping criteria in the list_for_each_table_entry macro (to be added
when all the callers are migrated)

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