OSHMEM: spml ikrit: fix mxm disconnect flow

Add out of band barrier before performing mxm disconnect.
It will make sure that every pe is ready to disconnect. Otherwise
bad things may happen.
(cherry picked from commit 3f7ed5654802d4b5921263daba7811f77b7751fc)
Merge pull request #130 from alex-mikheev/topic/spml_irkit_disconnect_fix-v1.8

OSHMEM: spml ikrit: fix mxm disconnect flow