lib/crc_kunit.c: update comment in crc_benchmark()

None of the CRC library functions use __pure anymore, so the comment in
crc_benchmark() is outdated.  But the comment was not really correct
anyway, since the CRC computation could (in principle) be optimized out
regardless of __pure.  Update the comment to have a proper explanation.

Link: https://lore.kernel.org/r/20250305015830.37813-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

lib/crc_kunit.c

@@ -226,8 +226,9 @@ crc_benchmark(struct kunit *test,
 	};
 	size_t len, i, j, num_iters;
 	/*
-	 * Some of the CRC library functions are marked as __pure, so use
-	 * volatile to ensure that all calls are really made as intended.
+	 * The CRC value that this function computes in a series of calls to
+	 * crc_func is never actually used, so use volatile to ensure that the
+	 * computations are done as intended and don't all get optimized out.
 	 */
 	volatile u64 crc = 0;
 	u64 t;