sysctl: drop sysctl_is_perm_empty_ctl_table

It is used only twice and those callers are simpler with
sysctl_is_perm_empty_ctl_header().
So use this sibling function.

This is part of an effort to constify definition of struct ctl_table.
For this effort the mutable member 'type' is moved from
struct ctl_table to struct ctl_table_header.
Unifying the macros sysctl_is_perm_empty_ctl_* makes this easier.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Joel Granados <j.granados@samsung.com>