crypto: rsa - restrict plaintext/ciphertext values more

SP 800-56Br2, Section 7.1.1 [1] specifies that:
1. If m does not satisfy 1 < m < (n – 1), output an indication that m is
out of range, and exit without further processing.

Similarly, Section 7.1.2 of the same standard specifies that:
1. If the ciphertext c does not satisfy 1 < c < (n – 1), output an
indication that the ciphertext is out of range, and exit without further
processing.

This range is slightly more conservative than RFC3447, as it also
excludes RSA fixed points 0, 1, and n - 1.

[1] https://doi.org/10.6028/NIST.SP.800-56Br2

Signed-off-by: Joachim Vandersmissen <git@jvdsn.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>