add consts where appropriate in fs/nls/*

Hi Alexander,

Attached patch adds 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.

Patch is compile-tested with .config with all in-tree filesystems selected.

Please send it Linuswards.

--
vda