fscrypt: correct collision claim for digested names

As I noted on the mailing list, it's easier than I originally thought to
create intentional collisions in the digested names.  Unfortunately it's
not too easy to solve this, so for now just fix the comment to not lie.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>