btrfs: push extent lock down in submit_one_async_extent

We don't need to include the time we spend in the allocator under our
extent lock protection, move it after the allocator and make sure we
lock the extent in the error case to ensure we're not clearing these
bits without the extent lock held.

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