udf: Fix NULL ptr deref when converting from inline format

udf_expand_file_adinicb() calls directly ->writepage to write data
expanded into a page. This however misses to setup inode for writeback
properly and so we can crash on inode->i_wb dereference when submitting
page for IO like:

  BUG: kernel NULL pointer dereference, address: 0000000000000158
  #PF: supervisor read access in kernel mode
...
  <TASK>
  __folio_start_writeback+0x2ac/0x350
  __block_write_full_page+0x37d/0x490
  udf_expand_file_adinicb+0x255/0x400 [udf]
  udf_file_write_iter+0xbe/0x1b0 [udf]
  new_sync_write+0x125/0x1c0
  vfs_write+0x28e/0x400

Fix the problem by marking the page dirty and going through the standard
writeback path to write the page. Strictly speaking we would not even
have to write the page but we want to catch e.g. ENOSPC errors early.

Reported-by: butt3rflyh4ck <[email protected]>
CC: [email protected]
Fixes: 52ebea7 ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks")
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Jan Kara <[email protected]>

fs/udf/inode.c

@@ -258,10 +258,6 @@ int udf_expand_file_adinicb(struct inode *inode)
 	char *kaddr;
 	struct udf_inode_info *iinfo = UDF_I(inode);
 	int err;
-	struct writeback_control udf_wbc = {
-		.sync_mode = WB_SYNC_NONE,
-		.nr_to_write = 1,
-	};
 
 	WARN_ON_ONCE(!inode_is_locked(inode));
 	if (!iinfo->i_lenAlloc) {
@@ -305,8 +301,10 @@ int udf_expand_file_adinicb(struct inode *inode)
 		iinfo->i_alloc_type = ICBTAG_FLAG_AD_LONG;
 	/* from now on we have normal address_space methods */
 	inode->i_data.a_ops = &udf_aops;
+	set_page_dirty(page);
+	unlock_page(page);
 	up_write(&iinfo->i_data_sem);
-	err = inode->i_data.a_ops->writepage(page, &udf_wbc);
+	err = filemap_fdatawrite(inode->i_mapping);
 	if (err) {
 		/* Restore everything back so that we don't lose data... */
 		lock_page(page);