fork: mv threads-max into kernel/fork.c

fork: mv threads-max into kernel/fork.c

make sysctl_max_threads static as it no longer needs to be exported into
sysctl.c.

This is part of a greater effort to move ctl tables into their
respective subsystems which will reduce the merge conflicts in
kernel/sysctl.c.

Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>