Legal characters in encrypted fscrypto (f2fs/ext4) filename?

[Eric Biggers]

Hello,

While reviewing the new filesystem encryption code, I was confused by the
intended set of legal characters in the printable form of an encrypted filename.

According to the actual code in fs/crypto/fname.c, the legal characters are:

	a-zA-Z0-9+,

Alternatively, according to the comment in the same file, the legal characters
are actually:

	a-zA-Z0-9+_

Still alternatively, according to the design document[1], the legal characters
are actually:

	a-zA-Z0-9+=

Which one is correct?

[1] https://docs.google.com/document/d/1ft26lUQyuSpiu6VleP70_npaWdRfXFoNnB8JYnykNTg/preview?pref=2&pli=1#