[ Upstream commit 2eb4942b ]

Currently used scalar multiplication algorithm (Matthieu Rivain, 2011)
have invalid values for scalar == 1, n-1, and for regularized version
n-2, which was previously not checked. Verify that they are not used as
private keys.
Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>