exfat: using hweight instead of internal logic

Replace the internal table lookup algorithm with the hweight
library, which has instruction set acceleration capabilities.

Use it to increase the length of a single calculation of
the exfat_find_free_bitmap function to the long type.

Signed-off-by: John Sanpe <sanpeqf@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>