Introduce new function clear_record_extent_bits(), which will clear bits
for given range and record the details about which ranges are cleared
and how many bytes in total it changes.

This provides the basis for later qgroup reserve codes.
Signed-off-by: NQu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>