Revert "f2fs: do not issue small discard commands during checkpoint"

Previously, we have two mechanisms to cache & submit small discards:

a) set max small discard number in /sys/fs/f2fs/vdb/max_small_discards,
and checkpoint will cache small discard candidates w/ configured maximum
number.

b) call FITRIM ioctl, also, checkpoint in f2fs_trim_fs() will cache small
discard candidates w/ configured discard granularity, but w/o limitation
of number. FSTRIM interface is asynchronized, so it won't submit discard
directly.

Finally, discard thread will submit them in background periodically.

However, after commit 9ac00e7c ("f2fs: do not issue small discard
commands during checkpoint"), the mechanism a) is broken, since no matter
how we configure the sysfs entry /sys/fs/f2fs/vdb/max_small_discards,
checkpoint will not cache small discard candidates any more.

echo 0 > /sys/fs/f2fs/vdb/max_small_discards
xfs_io -f /mnt/f2fs/file -c "pwrite 0 2m" -c "fsync"
xfs_io /mnt/f2fs/file -c "fpunch 0 4k"
sync
cat /proc/fs/f2fs/vdb/discard_plist_info |head -2

echo 100 > /sys/fs/f2fs/vdb/max_small_discards
rm /mnt/f2fs/file
xfs_io -f /mnt/f2fs/file -c "pwrite 0 2m" -c "fsync"
xfs_io /mnt/f2fs/file -c "fpunch 0 4k"
sync
cat /proc/fs/f2fs/vdb/discard_plist_info |head -2

Before the patch:
Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
  0         .       .       .       .       .       .       .       .
Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
  0         3       1       .       .       .       .       .       .

After the patch:
Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
  0         .       .       .       .       .       .       .       .
Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
  0         .       .       .       .       .       .       .       .

This patch reverts commit 9ac00e7c ("f2fs: do not issue small discard
commands during checkpoint") in order to fix this issue.

Fixes: 9ac00e7c ("f2fs: do not issue small discard commands during checkpoint")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>