IB/ehca: Reject dynamic memory add/remove when ehca adapter is present

Since the ehca device driver does not support dynamic memory add and
remove operations, the driver must explicitly reject such requests in
order to prevent unpredictable behaviors related to existing memory
regions that cover all of memory being used by InfiniBand protocols in
the kernel.

The solution (for now at least) is to add a memory notifier to the
ehca device driver and if a request for dynamic memory add or remove
comes in, ehca will always reject it.  The user can add or remove
memory by hot-removing the ehca adapter, performing the memory
operation, and then hot-adding the ehca adapter back.

Signed-off-by: Stefan Roscher <stefan.roscher@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>