FROMGIT: f2fs v2: factor out discard_cmd usage from general rb_tree use

I found a bug in the previous version and this patch fixes the gap from
upstream version.

Fixes: fcc385fd ("FROMGIT: f2fs: factor out discard_cmd usage from general rb_tree use")
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
(cherry picked from commit e39836183be8
https: //git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)
Change-Id: I4dbfb9f1f2cc956685a7c4de5fcfbba705c30cfb