FROMGIT: f2fs: show number of pending discard commands

This information can be used to check how much time we need to give to issue
all the discard commands.

Bug: 206863097
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit fc4ae5492ca4afd7a8a9d261f4908b09f221d314
 git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)
Change-Id: Ibd2f1d6c171f584ec9ca3817d9ea561db98f4693