+* automount: implement expire:
+ - set superblock timeout AUTOFS_DEV_IOCTL_TIMEOUT_CMD
+ - periodically run AUTOFS_DEV_IOCTL_EXPIRE_CMD
+ - every timeout/4 (original autofs logic)
+ - blocking, needs a thread
+ - run until -EAGAIN
+ - receive expire packet on pipe if kernel tells the timeout is over
+ - call umount
+ - answer expire packet on pipe with AUTOFS_DEV_IOCTL_{READY,FAIL}_CMD
+ - AUTOFS_DEV_IOCTL_EXPIRE_CMD returns