fscrypt: add comments that describe the HKDF info strings

fscrypt: add comments that describe the HKDF info strings

Each HKDF context byte is associated with a specific format of the
remaining part of the application-specific info string.  Add comments so
that it's easier to keep track of what these all are.

Link: https://lore.kernel.org/r/20200708215529.146890-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>