f2fs: convert to use MIN_DISCARD_GRANULARITY macro

Commit 1cd2e6d5 ("f2fs: define MIN_DISCARD_GRANULARITY macro")
introduce it, let's convert to use MIN_DISCARD_GRANULARITY macro.

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