xen/grant: introduce func gnttab_unmap_refs_sync()

There are several place using gnttab async unmap and wait for
completion, so move the common code to a function
gnttab_unmap_refs_sync().

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

diff --cc drivers/block/xen-blkback/blkback.c
Simple merge

diff --cc drivers/xen/gntdev.c
Simple merge

diff --cc drivers/xen/grant-table.c
Simple merge

diff --cc include/xen/grant_table.h
Simple merge