Running __mark_inode_dirty() against a swapcache page is illegal and will
oops.

I see a race in set_page_dirty() wherein it can be called with a PageSwapCache
page, but if the page is removed from swapcache after
__set_page_dirty_nobuffers() drops tree_lock(), we have the situation where
PageSwapCache() is false, but local variable `mapping' points at swapcache.

Handle that by checking for non-null mapping->host.  We don't care about the
page state at this point - we're only interested in the inode.



There is a converse case: what if a page is added to swapcache as we are
running set_page_dirty() against it?

In this case the page gets its PG_dirty flag set but it is not tagged as dirty
in the swapper_space radix tree.  The swap writeout code will handle this OK
and test_clear_page_dirty()'s call to
radix_tree_tag_clear(PAGECACHE_TAG_DIRTY) will silently have no effect.  The
only downside is that future radix-tree-based writearound won't notice that
such pages are dirty and swap IO scheduling will be a teensy bit worse.


The patch also fixes the (silly) testing of local variable `mapping' to see if
the page was truncated.  We should test page_mapping() for that.