proc: mark /proc/cmdline as permanent

/proc/cmdline is never removed, mark is as permanent for slightly faster
open and close.

Link: https://lkml.kernel.org/r/Y66xAveh2yUsP7m9@p183


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>