bcrypt: reject passwords longer than 72 bytes

By design, bcrypt only uses the first 72 bytes of a password when
generating a hash. Most implementations, including the reference one,
simply silently ignore any trailing input when provided passwords longer
than 72 bytes. This can cause confusion for users who expect the entire
password to be used to generate the hash.

In GenerateFromPassword, reject passwords longer than 72 bytes.
CompareHashAndPassword will still accept these passwords, since we
cannot break hashes that have already been stored.

Fixes golang/go#36546

Change-Id: I039addd2a2961a7fa9d1e4a3e892a9e3c8bf4c9a
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/450415
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Jason McNeil <jmcneil@x2studios.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>