eCryptfs: fix possible fault in ecryptfs_sync_page

This will avoid a possible fault in ecryptfs_sync_page().

In the function, eCryptfs calls sync_page() method of a lower filesystem
without checking its existence.  However, there are many filesystems that
don't have this method including network filesystems such as NFS, AFS, and
so forth.  They may fail when an eCryptfs page is waiting for lock.
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Acked-by: NMichael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/ecryptfs/mmap.c

@@ -834,7 +834,8 @@ static void ecryptfs_sync_page(struct page *page)
 		ecryptfs_printk(KERN_DEBUG, "find_lock_page failed\n");
 		return;
 	}
-	lower_page->mapping->a_ops->sync_page(lower_page);
+	if (lower_page->mapping->a_ops->sync_page)
+		lower_page->mapping->a_ops->sync_page(lower_page);
 	ecryptfs_printk(KERN_DEBUG, "Unlocking page with index = [0x%.16x]\n",
 			lower_page->index);
 	unlock_page(lower_page);