f2fs: define MIN_DISCARD_GRANULARITY macro

Do cleanup in f2fs_tuning_parameters() and __init_discard_policy(),
let's use macro instead of number.

Suggested-by: Chao Yu <chao@kernel.org>
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>