f2fs: export migration_granularity sysfs entry

Add one sysfs entry to control migration granularity of GC in large
section f2fs, it can be tuned to mitigate heavy overhead of migrating
huge number of blocks in large section.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>