block: add bdrv_refresh_perms() helper

Make separate function for common pattern.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201106124241.16950-5-vsementsov@virtuozzo.com>
[mreitz: Squashed in
https://lists.nongnu.org/archive/html/qemu-block/2020-11/msg00299.html

]
Signed-off-by: Max Reitz <mreitz@redhat.com>