Btrfs: use next_state in find_first_extent_bit

As next_state() is already defined to get the next state, use it in
find_first_extent_bit. No functional changes.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>