f2fs: multidevice: add stats in debugfs

This patch adds per-device stats in debugfs, the examples
are as below:

mkfs.f2fs -f -c /dev/vdc /dev/vdb
mount /dev/vdb /mnt/f2fs/
cat /sys/kernel/debug/f2fs/status

Multidevice stats:
  [seg:      inuse    dirty     full     free  prefree]
  #0             5        0        0     4007        0
  #1             1        0        0     8191        0

mkfs.f2fs -f -s 2 -c /dev/vdc /dev/vdb
mount /dev/vdb /mnt/f2fs
cat /sys/kernel/debug/f2fs/status

Multidevice stats:
  [seg:      inuse    dirty     full     free  prefree] [sec:      inuse    dirty     full     free  prefree]
  #0             5        0        0     4005        0                 5        0        0     2000        0
  #1             1        0        0     8191        0                 1        0        0     4095        0

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