ssh/agent: limit RSA key size on Add requests

parseRSAKey and parseRSACert build an rsa.PrivateKey directly from
the Add request body and then call priv.Precompute(). The CRT
coefficient recomputation in Precompute() is cubic in the size of
the prime factors, so accepting RSA keys with arbitrary modulus
and prime sizes can consume excessive CPU resources during a
single Add operation.

Add a checkRSAKeyParams helper that enforces the same bounds as
parseRSA in the ssh package (modulus <= 8192 bits, exponent <= 24
bits, exponent >= 3 and odd) plus a cap of 4096 bits on each prime
factor, and call it from both parseRSAKey and parseRSACert before
priv.Precompute().

The previous inline check on the exponent (BitLen > 30) is
subsumed by the new helper, which tightens it to BitLen > 24 for
consistency with parseRSA.

Change-Id: I6f0bbbfdc1db13d62a1f85e1d0e6ae517f000443
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/782421
Reviewed-by: Filippo Valsorda <filippo@golang.org>
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: Junyang Shao <shaojunyang@google.com>