sysctl: Replace nr_entries with ctl_table_size in new_links

The number of ctl_table entries (nr_entries) calculation was previously
based on the ctl_table_size and the sentinel element. Since the
sentinels have been removed, we remove the calculation and just use the
ctl_table_size from the ctl_table_header.

Signed-off-by: Joel Granados <j.granados@samsung.com>