PaX/GrSecurity Linux 3.8.y compat: Adopt ctl_table_no_const

The PaX team started constifying `struct ctl_table` as of their Linux
3.8.0 patchset. This lead to zfsonlinux/spl#225 and Gentoo bug #463012.

The PaX team suggested that we adopt ctl_table_no_const to deal with
this. While investigating our options, I learned that there is a
preprocessor directive called CONSTIFY_PLUGIN that we can use to detect
the presence of the PaX changes and adjust the code accordingly.
Consequently, we can switch to ctl_table_no_const and then use
CONSTIFY_PLUGIN to declare a typedef when the PaX plugin is not
available.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>