bitops: Optimize fns() for improved performance

The current fns() repeatedly uses __ffs() to find the index of the
least significant bit and then clears the corresponding bit using
__clear_bit(). The method for clearing the least significant bit can be
optimized by using word &= word - 1 instead.

Typically, the execution time of one __ffs() plus one __clear_bit() is
longer than that of a bitwise AND operation and a subtraction. To
improve performance, the loop for clearing the least significant bit
has been replaced with word &= word - 1, followed by a single __ffs()
operation to obtain the answer. This change reduces the number of
__ffs() iterations from n to just one, enhancing overall performance.

This modification significantly accelerates the fns() function in the
test_bitops benchmark, improving its speed by approximately 7.6 times.
Additionally, it enhances the performance of find_nth_bit() in the
find_bit benchmark by approximately 26%.

Before:
test_bitops: fns:             58033164 ns
find_nth_bit:                  4254313 ns,  16525 iterations

After:
test_bitops: fns:              7637268 ns
find_nth_bit:                  3362863 ns,  16501 iterations

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