f2fs: fix deadlock when flush inline data

Below backtrace info was reported by Yunlei He:

Call Trace:
 [<ffffffff817a9395>] schedule+0x35/0x80
 [<ffffffff817abb7d>] rwsem_down_read_failed+0xed/0x130
 [<ffffffff813c12a8>] call_rwsem_down_read_failed+0x18/0x
 [<ffffffff817ab1d0>] down_read+0x20/0x30
 [<ffffffffa02a1a12>] f2fs_evict_inode+0x242/0x3a0 [f2fs]
 [<ffffffff81217057>] evict+0xc7/0x1a0
 [<ffffffff81217cd6>] iput+0x196/0x200
 [<ffffffff812134f9>] __dentry_kill+0x179/0x1e0
 [<ffffffff812136f9>] dput+0x199/0x1f0
 [<ffffffff811fe77b>] __fput+0x18b/0x220
 [<ffffffff811fe84e>] ____fput+0xe/0x10
 [<ffffffff81097427>] task_work_run+0x77/0x90
 [<ffffffff81074d62>] exit_to_usermode_loop+0x73/0xa2
 [<ffffffff81003b7a>] do_syscall_64+0xfa/0x110
 [<ffffffff817acf65>] entry_SYSCALL64_slow_path+0x25/0x25

Call Trace:
 [<ffffffff817a9395>] schedule+0x35/0x80
 [<ffffffff81216dc3>] __wait_on_freeing_inode+0xa3/0xd0
 [<ffffffff810bc300>] ? autoremove_wake_function+0x40/0x4
 [<ffffffff8121771d>] find_inode_fast+0x7d/0xb0
 [<ffffffff8121794a>] ilookup+0x6a/0xd0
 [<ffffffffa02bc740>] sync_node_pages+0x210/0x650 [f2fs]
 [<ffffffff8122e690>] ? do_fsync+0x70/0x70
 [<ffffffffa02b085e>] block_operations+0x9e/0xf0 [f2fs]
 [<ffffffff8137b795>] ? bio_endio+0x55/0x60
 [<ffffffffa02b0942>] write_checkpoint+0x92/0xba0 [f2fs]
 [<ffffffff8117da57>] ? mempool_free_slab+0x17/0x20
 [<ffffffff8117de8b>] ? mempool_free+0x2b/0x80
 [<ffffffff8122e690>] ? do_fsync+0x70/0x70
 [<ffffffffa02a53e3>] f2fs_sync_fs+0x63/0xd0 [f2fs]
 [<ffffffff8129630f>] ? ext4_sync_fs+0xbf/0x190
 [<ffffffff8122e6b0>] sync_fs_one_sb+0x20/0x30
 [<ffffffff812002e9>] iterate_supers+0xb9/0x110
 [<ffffffff8122e7b5>] sys_sync+0x55/0x90
 [<ffffffff81003ae9>] do_syscall_64+0x69/0x110
 [<ffffffff817acf65>] entry_SYSCALL64_slow_path+0x25/0x25

With following excuting serials, we will set inline_node in inode page
after inode was unlinked, result in a deadloop described as below:
1. open file
2. write file
3. unlink file
4. write file
5. close file

Thread A				Thread B
 - dput
  - iput_final
   - inode->i_state |= I_FREEING
   - evict
    - f2fs_evict_inode
					 - f2fs_sync_fs
					  - write_checkpoint
					   - block_operations
					    - f2fs_lock_all (down_write(cp_rwsem))
     - f2fs_lock_op (down_read(cp_rwsem))
					    - sync_node_pages
					     - ilookup
					      - find_inode_fast
					       - __wait_on_freeing_inode
					         (wait on I_FREEING clear)

Here, we change to set inline_node flag only for linked inode for fixing.
Reported-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Tested-by: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: stable@vger.kernel.org # v4.6
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1546,6 +1546,7 @@ static int prepare_write_begin(struct f2fs_sb_info *sbi,
 		if (pos + len <= MAX_INLINE_DATA) {
 			read_inline_data(page, ipage);
 			set_inode_flag(F2FS_I(inode), FI_DATA_EXIST);
+			if (inode->i_nlink)
 				set_inline_node(ipage);
 		} else {
 			err = f2fs_convert_inline_page(&dn, page);