btrfs: factor out the ticket flush handling

We're going to make this logic a little more complicated for evict, so
factor the ticket flushing/waiting code out of __reserve_metadata_bytes.
This has no functional change.

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