f2fs: add NULL pointer check

Commit - fa9150a8 - replaces a call to generic_writepages() in
f2fs_write_data_pages() with write_cache_pages(), with a function pointer
argument pointing to routine: __f2fs_writepage.

  -> https://git.kernel.org/linus/fa9150a84ca333f68127097c4fa1eda4b3913a22

  This patch adds a NULL pointer check in f2fs_write_data_pages() to avoid
  a possible NULL pointer dereference, in case if - mapping->a_ops->writepage -
  is NULL.
Signed-off-by: P J P <ppandit@redhat.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/data.c

@@ -559,6 +559,10 @@ static int f2fs_write_data_pages(struct address_space *mapping,
 	int ret;
 	long excess_nrtw = 0, desired_nrtw;
 
+	/* deal with chardevs and other special file */
+	if (!mapping->a_ops->writepage)
+		return 0;
+
 	if (wbc->nr_to_write < MAX_DESIRED_PAGES_WP) {
 		desired_nrtw = MAX_DESIRED_PAGES_WP;
 		excess_nrtw = desired_nrtw - wbc->nr_to_write;