Hit a kernel oops on 2.6.7 kernel when doing direct I/O to hugetlb page.

The destructor of compound page was moved into page->mapping since 2.6.6.
It got interfered with set_page_dirty() for hugetlb page: an O_DIRECT read
into first tail page of the compound page will fool set_page_dirty() to
deference page->mapping->a_ops and then kernel oops.  Patch to fix the
oops.  We do just like what bio_set_pages_dirty() does.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>