OSHMEM: atomic mxm: fix mkey conversion

Correctly return mxm_empty_mem_key when shmem mkey is empty
(cherry picked from commit c76261da0749b2f93ee1a425baa4cc36ce2e1bef)
Merge pull request #133 from alex-mikheev/topic/mxm_atomic_fix-v1.8

OSHMEM: atomic mxm: fix mkey conversion