fscrypt: clean up base64 encoding/decoding

Some minor cleanups for the code that base64 encodes and decodes
encrypted filenames and long name digests:

- Rename "digest_{encode,decode}()" => "base64_{encode,decode}()" since
  they are used for filenames too, not just for long name digests.
- Replace 'while' loops with more conventional 'for' loops.
- Use 'u8' for binary data.  Keep 'char' for string data.
- Fully constify the lookup table (pointer was not const).
- Improve comment.

No actual change in behavior.

Signed-off-by: Eric Biggers <ebiggers@google.com>