OSHMEM: spml ikrit: fix mxm disconnect flow

@miked-mellanox @jladd-mlnx 

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 ompi/open-mpi@3f7ed5654802d4b5921263daba7811f77b7751fc)