add consts where appropriate in fs/nls/*

Add const modifiers to a few struct nls_table's member pointers in
include/linux/nls.h and adds a lot of const's in fs/nls/*.c files.

Resulting changes as visible by size:

   text    data     bss     dec     hex filename
 113612  481216    2368  597196   91ccc nls.org/built-in.o
 593548    3296     288  597132   91c8c nls/built-in.o

Apparently compiler managed to optimize code a bit better
because of const-ness.

No other changes are made.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>