mbedTLS NewCertificateSigningRequest fills wrong CSR length

[nileshvora-sls]

Problem
I was looking into creating csr for OpCSR for my ESP32 platform. However I was getting CSR length 0 when I call P256Keypair::NewCertificateSigningRequest. Then I looked into definition of it, it has written something like below.

    result = mbedtls_x509write_csr_pem(&csr, out_csr, length, CryptoRNG, nullptr);
    VerifyOrExit(result >= 0, error = CHIP_ERROR_INTERNAL);
    csr_length = static_cast<unsigned int>(result);

Proposed Solution
According to mbedtls API description mbedtls_x509write_csr_pem API return error code, not length. For length, we need to write something like below.

    result = mbedtls_x509write_csr_pem(&csr, out_csr, length, CryptoRNG, nullptr);
    VerifyOrExit(result >= 0, error = CHIP_ERROR_INTERNAL);
    csr_length = strlen((char *)out_csr);

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[nileshvora-sls]

CC @pan-apple @bzbarsky-apple

[pan-apple]

Thanks @nileshvora-sls , you are right about the return code. I'll push a fix for it.
The code was originally written to use mbedtls_x509write_csr() API, which returns the length. But in some iteration of the PR it got changed to PEM format.