ssh/knownhosts: verify declared key type matches decoded key

The parseLine function previously ignored the key type field (e.g.,
"ssh-rsa") in known_hosts entries, relying solely on the type
information embedded within the base64-encoded key blob.

OpenSSH's sshkey_read implementation explicitly verifies that the key
type declared in the text matches the type of the parsed key, returning
SSH_ERR_KEY_TYPE_MISMATCH if they differ.

This change adds a check to ensure the declared key type matches
key.Type(), returning an error for malformed lines where they diverge.

This issue was found during a security audit by NCC Group Cryptography
Services, sponsored by Teleport, and was assessed and is being fixed as
a non-security bug.

Change-Id: Id4f35c74055f5691088273630b50cdd02c81bfe9
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/782427
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
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>