btrfs: Implement find_first_clear_extent_bit

This function is very similar to find_first_extent_bit except that it
locates the first contiguous span of space which does not have bits set.
It's intended use is in the freespace trimming code.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>