f2fs: fix to set .discard_granularity correctly

commit 4f993264 ("f2fs: introduce discard_unit mount option") introduced
a bug, when we enable discard_unit=section option, it will set
.discard_granularity to BLKS_PER_SEC(), however discard granularity only
supports [1, 512], once section size is not equal to segment size, it will
cause issue_discard_thread() in DPOLICY_BG mode will not select discard entry
w/ any granularity to issue.

Fixes: 4f993264 ("f2fs: introduce discard_unit mount option")
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Yohan Joung <yohan.joung@sk.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>