Add /sys/kernel/debug/bdi/xxx/wb_stats to show per group writeback stats
of bdi.

Following domain hierarchy is tested:
                global domain (320G)
                /                 \
        cgroup domain1(10G)     cgroup domain2(10G)
                |                 |
bdi            wb1               wb2

/* per wb writeback info of bdi is collected */
cat wb_stats
WbCgIno:                    1
WbWriteback:                0 kB
WbReclaimable:              0 kB
WbDirtyThresh:              0 kB
WbDirtied:                  0 kB
WbWritten:                  0 kB
WbWriteBandwidth:      102400 kBps
b_dirty:                    0
b_io:                       0
b_more_io:                  0
b_dirty_time:               0
state:                      1

WbCgIno:                 4091
WbWriteback:             1792 kB
WbReclaimable:         820512 kB
WbDirtyThresh:        6004692 kB
WbDirtied:            1820448 kB
WbWritten:             999488 kB
WbWriteBandwidth:      169020 kBps
b_dirty:                    0
b_io:                       0
b_more_io:                  1
b_dirty_time:               0
state:                      5

WbCgIno:                 4131
WbWriteback:             1120 kB
WbReclaimable:         820064 kB
WbDirtyThresh:        6004728 kB
WbDirtied:            1822688 kB
WbWritten:            1002400 kB
WbWriteBandwidth:      153520 kBps
b_dirty:                    0
b_io:                       0
b_more_io:                  1
b_dirty_time:               0
state:                      5

[shikemeng@huaweicloud.com: fix build problems]
  Link: https://lkml.kernel.org/r/20240423034643.141219-4-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20240423034643.141219-3-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Brian Foster <bfoster@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: David Sterba <dsterba@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>