f2fs: avoid mismatching block range for discard

This patch skip discard block range smaller than trim_minlen,
and can not be merged by neighbour

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