libcli/smb: let smb2_signing_decrypt_pdu() cope with gnutls_aead_cipher_decrypt(...

libcli/smb: let smb2_signing_decrypt_pdu() cope with gnutls_aead_cipher_decrypt() ptext_len bug

The initial implementation of gnutls_aead_cipher_decrypt() had a bug and
used:
    *ptext_len = ctext_len;
instead of:
    *ptext_len = ctext_len - tag_size;

This got fixed with gnutls 3.5.2.

As we only require gnutls 3.4.7 we need to cope with this...

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14968

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Wed Feb  2 18:29:08 UTC 2022 on sn-devel-184