f2fs: support to show curseg.next_blkoff in debugfs

f2fs: support to show curseg.next_blkoff in debugfs

cat /sys/kernel/debug/f2fs/status

Main area: 17 segs, 17 secs 17 zones
    TYPE           blkoff    segno    secno   zoneno  dirty_seg   full_seg  valid_blk
  - COLD   data:        0        4        4        4          0          0          0
  - WARM   data:        0        7        7        7          0          0          0
  - HOT    data:        1        5        5        5          2          0        512
  - Dir   dnode:        3        0        0        0          1          0          2
  - File  dnode:        0        1        1        1          0          0          0
  - Indir nodes:        0        2        2        2          0          0          0
  - Pinned file:        0       -1       -1       -1
  - ATGC   data:        0       -1       -1       -1

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