The only caller is bdrv_set_locked(), and it ignores the value.

Callees always return 0, except for FreeBSD's cdrom_set_locked(),
which returns -ENOTSUP when the device is in a terminally wedged
state.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>