fix some conversion overflows

Fix page index to offset conversion overflows in buffer layer, ecryptfs,
and ocfs2.

It would be nice to convert the whole tree to page_offset, but for now
just fix the bugs.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Michael Halcrow <mhalcrow@us.ibm.com>
Cc: Mark Fasheh <mark.fasheh@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ocfs2/mmap.c

@@ -89,7 +89,7 @@ static int __ocfs2_page_mkwrite(struct inode *inode, struct buffer_head *di_bh,
 {
 	int ret;
 	struct address_space *mapping = inode->i_mapping;
-	loff_t pos = page->index << PAGE_CACHE_SHIFT;
+	loff_t pos = page_offset(page);
 	unsigned int len = PAGE_CACHE_SIZE;
 	pgoff_t last_index;
 	struct page *locked_page = NULL;