zero out last page for llseek/write

When one llseek's past the end of the file and then writes, every page past
the previous end of the file should be cleared.  Trevor found that the code,
as is, does not assure that the very last page is always cleared.  This patch
takes care of that.
Signed-off-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

@@ -422,6 +422,8 @@ static int ecryptfs_prepare_write(struct file *file, struct page *page,
 				goto out;
 			}
 		}
+		if (end_of_prev_pg_pos + 1 > i_size_read(page->mapping->host))
+			zero_user_page(page, 0, PAGE_CACHE_SIZE, KM_USER0);
 	}
 out:
 	return rc;