msm: ion: fix ion cache maintenance issue with highmem

ion cache maintenance code while doing cache maintenance
on each sg is only considering page length which may not
always be correct. An sg node can have more than page
length contiguous pages.

Update the code to properly take care of any length sg.

Change-Id: Ifdf8a8fa26abfa5bd47185a1039880306246d587
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>