sysctl: Wrap do_proc_douintvec with the public function proc_douintvec_conv

Make do_proc_douintvec static and export proc_douintvec_conv wrapper
function for external use. This is to keep with the design in sysctl.c.
Update fs/pipe.c to use the new public API.

Signed-off-by: Joel Granados <joel.granados@kernel.org>