nilfs2: fix null pointer dereference at nilfs_segctor_do_construct()

After commit c3aab9a ("mm/filemap.c: don't initiate writeback if
mapping has no dirty pages"), the following null pointer dereference has
been reported on nilfs2:

  BUG: kernel NULL pointer dereference, address: 00000000000000a8
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP PTI
  ...
  RIP: 0010:percpu_counter_add_batch+0xa/0x60
  ...
  Call Trace:
    __test_set_page_writeback+0x2d3/0x330
    nilfs_segctor_do_construct+0x10d3/0x2110 [nilfs2]
    nilfs_segctor_construct+0x168/0x260 [nilfs2]
    nilfs_segctor_thread+0x127/0x3b0 [nilfs2]
    kthread+0xf8/0x130
    ...

This crash turned out to be caused by set_page_writeback() call for
segment summary buffers at nilfs_segctor_prepare_write().

set_page_writeback() can call inc_wb_stat(inode_to_wb(inode),
WB_WRITEBACK) where inode_to_wb(inode) is NULL if the inode of
underlying block device does not have an associated wb.

This fixes the issue by calling inode_attach_wb() in advance to ensure
to associate the bdev inode with its wb.

Fixes: c3aab9a ("mm/filemap.c: don't initiate writeback if mapping has no dirty pages")
Reported-by: Walton Hoops <[email protected]>
Reported-by: Tomas Hlavaty <[email protected]>
Reported-by: ARAI Shun-ichi <[email protected]>
Reported-by: Hideki EIRAKU <[email protected]>
Signed-off-by: Ryusuke Konishi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Ryusuke Konishi <[email protected]>
Cc: <[email protected]>	[5.4+]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: konis

fs/nilfs2/segment.c

@@ -2780,6 +2780,8 @@ int nilfs_attach_log_writer(struct super_block *sb, struct nilfs_root *root)
 	if (!nilfs->ns_writer)
 		return -ENOMEM;
 
+	inode_attach_wb(nilfs->ns_bdev->bd_inode, NULL);
+
 	err = nilfs_segctor_start_thread(nilfs->ns_writer);
 	if (err) {
 		kfree(nilfs->ns_writer);