pmem: Make request/release region return a value

the request_region/release_region call backs now return
an error code. This way if the request fails the allocation
can fail as well.

Change-Id: I817c65a43935fb41176ad03f5a7672fb5640084e
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Merge "pmem: Make request/release region return a value" into jellybean