procfs: constify function pointer tables

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-By: David Howells <dhowells@redhat.com>
Acked-by: Bryan Wu <bryan.wu@analog.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>