btrfs: only run delayed refs once before committing

We try to pre-flush the delayed refs when committing, because we want to
do as little work as possible in the critical section of the transaction
commit.

However doing this twice can lead to very long transaction commit delays
as other threads are allowed to continue to generate more delayed refs,
which potentially delays the commit by multiple minutes in very extreme
cases.

So simply stick to one pre-flush, and then continue the rest of the
transaction commit.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>