blkdeactivate: fix endless loop if device(s) given and unable to umount/deactivate

blkdeactivate: fix endless loop if device(s) given and unable to umount/deactivate

The blkdeactivate script iterates over the list of devices if they're
given as an argument and it tries to umount/deactivate them one by one.

This iteration failed to proceed if any of the umount/deactivation
was unsuccessful - there was a missing "shift" call to move to the
next argument (device) for processing. As a result of this, the same
device was tried again and again, causing an endless loop, never
proceeding to the next device given.