IB/ehca: Serialize MR alloc and MR free hvCalls

Some firmware levels exhibit a race condition between H_ALLOC_RESOURCE(MR)
and H_FREE_RESOURCE(MR).  Work around this problem by locking these hvCalls
against each other.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>