[SCSI] sg and st unmap_user_pages allow PageReserved

2.6.15-rc1 made sg's st_unmap_user_pages and st's sgl_unmap_user_pages
BUG on a PageReserved page. But that's wrong: they could be unmapping
the ZERO_PAGE, which is marked PG_reserved; and perhaps others (while
get_user_pages is still permitted on VM_PFNMAP areas - that may change).

More change is needed here: sg claims to dirty even pages written from,
and st claims not to dirty even pages read into; and SetPageDirty is not
adequate for this nowadays. Fixes to those follow in a later patch: for
the moment just fix the 2.6.15 regression.

Signed-off-by: Hugh Dickins <[e-mail]>
Acked-by: Nick Piggin <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

drivers/scsi/sg.c

···
 	for (i=0; i < nr_pages; i++) {
 		struct page *page = sgl[i].page;
 
-		/* XXX: just for debug. Remove when PageReserved is removed */
-		BUG_ON(PageReserved(page));
 		if (dirtied)
 			SetPageDirty(page);
 		/* unlock_page(page); */

drivers/scsi/st.c

···
 	for (i=0; i < nr_pages; i++) {
 		struct page *page = sgl[i].page;
 
-		/* XXX: just for debug. Remove when PageReserved is removed */
-		BUG_ON(PageReserved(page));
 		if (dirtied)
 			SetPageDirty(page);
 		/* FIXME: cache flush missing for rw==READ