libmedtls: mpi_miller_rabin: increase count limit

Increase the count limit when generating the witness in the Rabin-Miller
primality test. The previous number 30 was too low to reliably detect
000000022770A7DC599BC90B2FF981CCB5CF05703344C8F350418AAD as a prime
number.

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
[jf: rebased onto mbedtls-2.22.0]
[jf: rebased onto mbedtls-2.27.0]
Signed-off-by: Jerome Forissier <jerome@forissier.org>

lib/libmbedtls/mbedtls/library/bignum.c

@@ -2747,7 +2747,7 @@ static int mpi_miller_rabin( const mbedtls_mpi *X, size_t rounds,
                 A.p[A.n - 1] &= ( (mbedtls_mpi_uint) 1 << ( k - ( A.n - 1 ) * biL - 1 ) ) - 1;
             }
 
-            if (count++ > 30) {
+            if (count++ > 300) {
                 ret = MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
                 goto cleanup;
             }