block: support hierarchical mount/umount

This is needed when e.g. sda1 needs to be mounted to /mnt and sda2
/mnt/srv.  We need to make sure sda1 was mounted first.

"devices" is now an ordered list with elements pointing to both "struct
probe_info" and "struct mount".  The order is firstly on presence of
mount info, then strlen(mount.target), then strcmp(probe_info.dev)

Resolves FS#2214

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>