Change the grant table interface so that gnttab_end_foreign_access_ref returns

Change the grant table interface so that gnttab_end_foreign_access_ref returns
an error code if the page is still in use, and gnttab_end_foreign_access takes
a page to free.  This allows us to cope with frontends wanting to end access
while backends are still using pages by, eventually, placing the page and
grant table entry on a list, for freeing later.  At the moment, the page and
grant table entry are leaked, with a diagnostic.

Tidied up netfront.c's clean-up code.

Signed-off-by: Ewan Mellor <ewan@xensource.com>