Add a new structure, extent_change_set, to record how many bytes are
changed in one set/clear_extent_bits() operation, with detailed changed
ranges info.

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