Btrfs: fix find_free_dev_extent() malfunction in case device tree has hole

If device tree has hole, find_free_dev_extent() cannot find available
address properly.

The problem can be reproduce by following script.

    mntpath=/btrfs
    loopdev=/dev/loop0
    filepath=/home/forrest/image

    umount $mntpath
    losetup -d $loopdev
    truncate --size 100g $filepath
    losetup $loopdev $filepath
    mkfs.btrfs -f $loopdev
    mount $loopdev $mntpath

    # make device tree with one big hole
    for i in `seq 1 1 100`; do
        fallocate -l 1g $mntpath/$i
    done
    sync
    for i in `seq 1 1 95`; do
        rm $mntpath/$i
    done
    sync

    # wait cleaner thread remove unused block group
    sleep 300

    fallocate -l 1g $mntpath/aaa

    # failed to allocate new chunk
    fallocate -l 1g $mntpath/bbb

Above script will make device tree with one big hole, and can only allocate
just one chunk in a transaction, so failed to allocate new chunk for $mntpath/bbb

    item 8 key (1 DEV_EXTENT 2185232384) itemoff 15859 itemsize 48
        dev extent chunk_tree 3
        chunk objectid 256 chunk offset 106292051968 length 1073741824
    item 9 key (1 DEV_EXTENT 104190705664) itemoff 15811 itemsize 48
        dev extent chunk_tree 3
        chunk objectid 256 chunk offset 103108575232 length 1073741824

Signed-off-by: Forrest Liu <forrestl@synology.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>