f2fs: flush stale issued discard candidates

Sometimes, I could observe # of issuing_discard to be 1 which blocks background
jobs due to is_idle()=false.
The only way to get out of it was to trigger gc_urgent. This patch avoids that
by checking any candidates as done in the list.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>