ssh: cap total userauth attempts per server connection

serverAuthenticate only bounded real failures via MaxAuthTries.
PartialSuccessError responses and the publickey isQuery short-circuit
both kept the loop running without incrementing authFailures, so a
client could keep the server processing SSH_MSG_USERAUTH_REQUEST
messages indefinitely on a single connection.

Add an unconditional cap, maxAuthServerAttempts = 128, on the total
number of userauth requests handled per connection. The counter is
incremented at the top of the loop before method dispatch, so every
method and every isQuery / partial-success path counts. When the cap
is exceeded the server sends SSH_MSG_DISCONNECT with reason 2 ("too
many authentication attempts"), mirroring the MaxAuthTries handling.
The bound is well below OpenSSH's hard cap of 1024 but above any
realistic multi-step auth flow.

Change-Id: I56779fc55cd00ddfd32ec938f8de3a49be0145dc
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781903
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>