f2fs: fix incorrect bimodal calculation

In update_sit_info, we use div_u64 to handle 'u64 divide u64' case, but
div_u64 can only handle 32-bits divisor, so our divisor with u64 type
passed to div_u64 will overflow, result in the wrong calculation when
show debug info of f2fs as below:

BDF: 464, avg. vblocks: 23509
(BDF should never exceed 100)

So change to use div64_u64 to handle this case correctly.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>