Btrfs: do not start delalloc inodes during sync

btrfs_start_delalloc_inodes will just walk the list of delalloc inodes and
start writing them out, but it doesn't splice the list or anything so as
long as somebody is doing work on the box you could end up in this section
_forever_.  So just remove it, it's not needed anyway since sync will start
writeback on all inodes anyway, all we need to do is wait for ordered
extents and then we can commit the transaction.  In my horrible torture test
sync goes from taking 4 minutes to about 1.5 minutes.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>