Btrfs: hold the ordered operations mutex when waiting on ordered extents

We need to hold the ordered_operations mutex while waiting on ordered extents
since we splice and run the ordered extents list.  We need to make sure anybody
else who wants to wait on ordered extents does actually wait for them to be
completed.  This will keep us from bailing out of flushing in case somebody is
already waiting on ordered extents to complete.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>