exfat: using ffs instead of internal logic

Replaced the internal table lookup algorithm with ffs of
the bitops library with better performance.

Use it to increase the single processing length of the
exfat_find_free_bitmap function, from single-byte search to long type.

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