drivers: entropy: fix native_posix driver for more than 3 byte requests

Fix the native_posix fake entropy driver for more than 3 byte requests,
and specially for native_sim//64 builds.

The host random() provides a number between 0 and 2**31-1 (INT_MAX),
so bit 32 was always 0.
So when filling a buffer with more than 3 bytes we would be filling
each 4th byte with a byte which always had its MSbit as 0.

For LP64 native_sim//64 builds, this was even worse, as the driver had
another bug where we assumed random() returned the whole long filled,
and therefore all 4 upper bytes would be 0.

Signed-off-by: Alberto Escolar Piedras <[email protected]>
(cherry picked from commit b3407f0)

Author: aescolar

drivers/entropy/fake_entropy_native_posix.c

@@ -41,7 +41,10 @@ static int entropy_native_posix_get_entropy(const struct device *dev,
 		 */
 		long value = nsi_host_random();
 
-		size_t to_copy = MIN(length, sizeof(long int));
+		/* The host random() provides a number between 0 and 2**31-1. Bit 32 is always 0.
+		 * So let's just use the lower 3 bytes discarding the upper 7 bits
+		 */
+		size_t to_copy = MIN(length, 3);
 
 		memcpy(buffer, &value, to_copy);
 		buffer += to_copy;