powerpc/crypto: Use cheaper random numbers for crc-vpmsum self-test

This code was filling a 64K buffer from /dev/urandom in order to
compute a CRC over (on average half of) it by two different methods,
comparing the CRCs, and repeating.

This is not a remotely security-critical application, so use the far
faster and cheaper prandom_u32() generator.

And, while we're at it, only fill as much of the buffer as we plan to use.
Signed-off-by: George Spelvin <lkml@sdf.org>
Acked-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/crypto/crc-vpmsum_test.c

@@ -78,16 +78,12 @@ static int __init crc_test_init(void)
 
 		pr_info("crc-vpmsum_test begins, %lu iterations\n", iterations);
 		for (i=0; i<iterations; i++) {
-			size_t len, offset;
+			size_t offset = prandom_u32_max(16);
+			size_t len = prandom_u32_max(MAX_CRC_LENGTH);
 
-			get_random_bytes(data, MAX_CRC_LENGTH);
-			get_random_bytes(&len, sizeof(len));
-			get_random_bytes(&offset, sizeof(offset));
-
-			len %= MAX_CRC_LENGTH;
-			offset &= 15;
 			if (len <= offset)
 				continue;
+			prandom_bytes(data, len);
 			len -= offset;
 
 			crypto_shash_update(crct10dif_shash, data+offset, len);