vfs: Don't call attach_nobh_buffers() with an empty list

This is a modification of a patch by Bill Pemberton <wfp5p@virginia.edu>

nobh_write_end() could call attach_nobh_buffers() with head == NULL.
This would result in a trap when attach_nobh_buffers() attempted to
access bh->b_this_page.

This can be illustrated by running the writev01 testcase from LTP on jfs.

This error was introduced by commit 5b41e74 "vfs: fix data leak in
nobh_write_end()".  That patch did not take into account that if
PageMappedToDisk() is true upon entry to nobh_write_begin(), then no
buffers will be allocated for the page.  In that case, we won't have to
worry about a failed write leaving unitialized data in the page.

Of course, head != NULL implies !page_has_buffers(page), so no need to
test both.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Cc: Bill Pemberton <wfp5p@virginia.edu>
Cc: Dmitri Monakhov <dmonakhov@openvz.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/buffer.c

@@ -2688,7 +2688,7 @@ int nobh_write_end(struct file *file, struct address_space *mapping,
 	struct buffer_head *bh;
 	BUG_ON(fsdata != NULL && page_has_buffers(page));
 
-	if (unlikely(copied < len) && !page_has_buffers(page))
+	if (unlikely(copied < len) && head)
 		attach_nobh_buffers(page, head);
 	if (page_has_buffers(page))
 		return generic_write_end(file, mapping, pos, len,