OSHMEM: sshmem mmap: use MAP_PRIVATE instead of MAP_SHARED

It looks like using MAP_PRIVATE instead of MAP_SHARED greatly
speeds up infiniband memory registration.

Change-Id: Id7089f58458ef8fff4034a2c4707d31f7e8b6694
(cherry picked from commit 89535a32726450a92fa134cb32d82f0362c9877b)
Merge pull request #10 from miked-mellanox/topic/fix-mmap-mode

OSHMEM: fix memheap/sshmem/mmap to use MAP_PRIVATE instead of SHARED to speedup registration
RM-approved