IB/hfi1: Prevent unpinning of wrong pages

The routine used by the SDMA cache to handle already
cached nodes can extend an already existing node.

In its error handling code, the routine will unpin pages
when not all pages of the buffer extension were pinned.

There was a bug in that part of the routine, which would
mistakenly unpin pages from the original set rather than
the newly pinned pages.

This commit fixes that bug by offsetting the page array
to the proper place pointing at the beginning of the newly
pinned pages.

Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>