sysctl: replace child with an enumeration

sysctl: replace child with an enumeration

This is part of the effort to remove the empty element at the end of
ctl_table structs. "child" was a deprecated elem in this struct and was
being used to differentiate between two types of ctl_tables: "normal"
and "permanently emtpy".

What changed?:
* Replace "child" with an enumeration that will have two values: the
  default (0) and the permanently empty (1). The latter is left at zero
  so when struct ctl_table is created with kzalloc or in a local
  context, it will have the zero value by default. We document the
  new enum with kdoc.
* Remove the "empty child" check from sysctl_check_table
* Remove count_subheaders function as there is no longer a need to
  calculate how many headers there are for every child
* Remove the recursive call to unregister_sysctl_table as there is no
  need to traverse down the child tree any longer
* Add a new SYSCTL_PERM_EMPTY_DIR binary flag
* Remove the last remanence of child from partport/procfs.c

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