f2fs: Fix type of section block count variables

Commit da52f8ad ("f2fs: get the right gc victim section when section
has several segments") added code to count blocks of each section using
variables with type 'unsigned short', which has 2 bytes size in many
systems. However, the counts can be larger than the 2 bytes range and
type conversion results in wrong values. Especially when the f2fs
sections have blocks as many as USHRT_MAX + 1, the count is handled as 0.
This triggers eternal loop in init_dirty_segmap() at mount system call.
Fix this by changing the type of the variables to block_t.

Fixes: da52f8ad ("f2fs: get the right gc victim section when section has several segments")
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>