When testing f2fs with inline_dentry option, generic/342 reports:
VFS: Busy inodes after unmount of dm-0. Self-destruct in 5 seconds.  Have a nice day...

After rmmod f2fs module, kenrel shows following dmesg:
 =============================================================================
 BUG f2fs_inode_cache (Tainted: G           O   ): Objects remaining in f2fs_inode_cache on __kmem_cache_shutdown()
 -----------------------------------------------------------------------------

 Disabling lock debugging due to kernel taint
 INFO: Slab 0xf51ca0e0 objects=22 used=1 fp=0xd1e6fc60 flags=0x40004080
 CPU: 3 PID: 7455 Comm: rmmod Tainted: G    B      O    4.6.0-rc4+ #16
 Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
  00000086 00000086 d062fe18 c13a83a0 f51ca0e0 d062fe38 d062fea4 c11c7276
  c1981040 f51ca0e0 00000016 00000001 d1e6fc60 40004080 656a624f 20737463
  616d6572 6e696e69 6e692067 66326620 6e695f73 5f65646f 68636163 6e6f2065
 Call Trace:
  [<c13a83a0>] dump_stack+0x5f/0x8f
  [<c11c7276>] slab_err+0x76/0x80
  [<c11cbfc0>] ? __kmem_cache_shutdown+0x100/0x2f0
  [<c11cbfc0>] ? __kmem_cache_shutdown+0x100/0x2f0
  [<c11cbfe5>] __kmem_cache_shutdown+0x125/0x2f0
  [<c1198a38>] kmem_cache_destroy+0x158/0x1f0
  [<c176b43d>] ? mutex_unlock+0xd/0x10
  [<f8f15aa3>] exit_f2fs_fs+0x4b/0x5a8 [f2fs]
  [<c10f596c>] SyS_delete_module+0x16c/0x1d0
  [<c1001b10>] ? do_fast_syscall_32+0x30/0x1c0
  [<c13c59bf>] ? __this_cpu_preempt_check+0xf/0x20
  [<c10afa7d>] ? trace_hardirqs_on_caller+0xdd/0x210
  [<c10ad50b>] ? trace_hardirqs_off+0xb/0x10
  [<c1001b81>] do_fast_syscall_32+0xa1/0x1c0
  [<c176d888>] sysenter_past_esp+0x45/0x74
 INFO: Object 0xd1e6d9e0 @offset=6624
 kmem_cache_destroy f2fs_inode_cache: Slab cache still has objects
 CPU: 3 PID: 7455 Comm: rmmod Tainted: G    B      O    4.6.0-rc4+ #16
 Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
  00000286 00000286 d062fef4 c13a83a0 f174b000 d062ff14 d062ff28 c1198ac7
  c197fe18 f3c5b980 d062ff20 000d04f2 d062ff0c d062ff0c d062ff14 d062ff14
  f8f20dc0 fffffff5 d062e000 d062ff30 f8f15aa3 d062ff7c c10f596c 73663266
 Call Trace:
  [<c13a83a0>] dump_stack+0x5f/0x8f
  [<c1198ac7>] kmem_cache_destroy+0x1e7/0x1f0
  [<f8f15aa3>] exit_f2fs_fs+0x4b/0x5a8 [f2fs]
  [<c10f596c>] SyS_delete_module+0x16c/0x1d0
  [<c1001b10>] ? do_fast_syscall_32+0x30/0x1c0
  [<c13c59bf>] ? __this_cpu_preempt_check+0xf/0x20
  [<c10afa7d>] ? trace_hardirqs_on_caller+0xdd/0x210
  [<c10ad50b>] ? trace_hardirqs_off+0xb/0x10
  [<c1001b81>] do_fast_syscall_32+0xa1/0x1c0
  [<c176d888>] sysenter_past_esp+0x45/0x74

The reason is: in recovery flow, we use delayed iput mechanism for directory
which has recovered dentry block. It means the reference of inode will be
held until last dirty dentry page being writebacked.

But when we mount f2fs with inline_dentry option, during recovery, dirent
may only be recovered into dir inode page rather than dentry page, so there
are no chance for us to release inode reference in ->writepage when
writebacking last dentry page.

We can call paired iget/iput explicityly for inline_dentry case, but for
non-inline_dentry case, iput will call writeback_single_inode to write all
data pages synchronously, but during recovery, ->writepages of f2fs skips
writing all pages, result in losing dirent.

This patch fixes this issue by obsoleting old mechanism, and introduce a
new dir_list to hold all directory inodes which has recovered datas until
finishing recovery.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>