block: Don't let locked flag prevent medium load

Commit aea2a33c made bdrv_eject() obey the locked flag.  Correct for
medium eject (eject_flag set), incorrect for medium load (eject_flag
clear).  See MMC-5 Table 341 "Actions for Lock/Unlock/Eject".

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