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>