when decide that data is no longer needed, there are two cases:
-if data neighbours previous block of "no needed" data, extend bunch
block(it holds begining and size of concequent "no needed" data) by
length of curent block and go next.
-if data not neighbours bunch block(or bunch block size will be bigger
than MAX_BUNCH_SIZE), than we punch bunch block and set bunch block
to curent block.

in the end make cleanup to punch last bunch block.

changes in v1:
punch_hole takes whole page_read
make restriction more precise
Signed-off-by: Tikhomirov Pavel <snorcht@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>