f2fs: combine huge num of discard rb tree consistence checks

Came across a hungtask caused by huge number of rb tree traversing
during adding discard addrs in cp. This patch combine these consistence
checks and move it to discard thread.

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