Don't truncate the input when decrypting in pkeyutl

Don't truncate the input when decrypting in pkeyutl

The pkeyutl app was truncating the input file for decryption leading to
incorrect results. This was probably ok historically when RSA was being
used for decryption which has short maximum sizes. This is not ok with SM2.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21272)

(cherry picked from commit 849450746f38a5658ef783abb0a8c79ae2861464)