There's no need to keep the foreign access in a grant if it is not
persistently mapped by the backend. This allows us to free grants that
are not mapped by the backend, thus preventing blkfront from hoarding
all grants.

The main effect of this is that blkfront will only persistently map
the same grants as the backend, and it will always try to use grants
that are already mapped by the backend. Also the number of persistent
grants in blkfront is the same as in blkback (and is controlled by the
value in blkback).
Signed-off-by: NRoger Pau Monné <roger.pau@citrix.com>
Reviewed-by: NDavid Vrabel <david.vrabel@citrix.com>
Acked-by: NMatt Wilson <msw@amazon.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>