lib/test_bitops: Add benchmark test for fns()

Introduce a benchmark test for the fns(). It measures the total time
taken by fns() to process 10,000 test data generated using
get_random_bytes() for each n in the range [0, BITS_PER_LONG).

example:
test_bitops: fns:             7637268 ns

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Rasmus Villemoes <linux@rasmusvillemoes.dk>
CC: David Laight <David.Laight@aculab.com>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>