btrfs: only use ->max_extent_size if it is set in the bitmap

While adding self tests for my space index change I was hitting a
problem where the space indexed tree wasn't returning the expected
->max_extent_size.  This is because we will skip searching any entry
that doesn't have ->bytes >= the amount of bytes we want.  However we'll
still set the max_extent_size based on that entry.  The problem is if we
don't search the bitmap we won't have ->max_extent_size set properly, so
we can't really trust it.

This doesn't really result in a problem per-se, it can just result in us
not finding contiguous area that may exist.  Fix the max_extent_size
helper to return ->bytes if ->max_extent_size isn't set, and add a big
comment explaining why we're doing this.

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