Revert behavior for zero-sized regions.
Revert behavior for zero-sized regions.

The size of memory region registered by ibv_reg_mr has to be at least of
length one. I.e. the previous solution depended on implementation which
is bad. Thus revert to create ibv_mr struct without any corresponding
registered memory region.

pjgeorg/pMR,pjgeorg/pMR,pjgeorg/pMR