btrfs: always wait on ordered extents at fsync time

btrfs: always wait on ordered extents at fsync time

There's a priority inversion that exists currently with btrfs fsync.  In
some cases we will collect outstanding ordered extents onto a list and
only wait on them at the very last second.  However this "very last
second" falls inside of a transaction handle, so if we are in a lower
priority cgroup we can end up holding the transaction open for longer
than needed, so if a high priority cgroup is also trying to fsync()
it'll see latency.

Signed-off-by: Josef Bacik <[email protected]>
Reviewed-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>