reiserfs: constify function pointer tables

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Acked-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>