f2fs: fix to clear FI_VOLATILE_FILE correctly

Thread A			Thread B
- f2fs_release_file
 - clear_inode_flag(FI_VOLATILE_FILE)
				- wb_writeback
				 - writeback_sb_inodes
				  - __writeback_single_inode
				   - do_writepages
				    - f2fs_write_data_pages
				     - __write_data_page
				     all volatile file's pages
				     are writebacked to storage
 - set_inode_flag(FI_DROP_CACHE)
 - filemap_fdatawrite

There is a hole that mm can flush all dirty pages of volatile file as
inode is not tagged with both FI_VOLATILE_FILE and FI_DROP_CACHE flags,
we should never writeback the page #0 and also it's unneeded to writeback
other pages.

This patch adjusts to relocate clear_inode_flag(FI_VOLATILE_FILE), so that
FI_VOLATILE_FILE flag can be remained before all dirty pages were dropped
to avoid issue.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -1564,11 +1564,11 @@ static int f2fs_release_file(struct inode *inode, struct file *filp)
 	if (f2fs_is_atomic_file(inode))
 		f2fs_drop_inmem_pages(inode);
 	if (f2fs_is_volatile_file(inode)) {
-		clear_inode_flag(inode, FI_VOLATILE_FILE);
-		stat_dec_volatile_write(inode);
 		set_inode_flag(inode, FI_DROP_CACHE);
 		filemap_fdatawrite(inode->i_mapping);
 		clear_inode_flag(inode, FI_DROP_CACHE);
+		clear_inode_flag(inode, FI_VOLATILE_FILE);
+		stat_dec_volatile_write(inode);
 	}
 	return 0;
 }