proc_sysctl: move helper which creates required subdirectories

proc_sysctl: move helper which creates required subdirectories

Move the code which creates the subdirectories for a ctl table
into a helper routine so to make it easier to review. Document
the goal.

This creates no functional changes.

Reviewed-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>