IB/ehca: Fall back to vmalloc() for big allocations

In case of large queue pairs there is the possibillity of allocation
failures due to memory fragmentation when using kmalloc().  To ensure
the memory is allocated even if kmalloc() can not find chunks which
are big enough, we fall back to allocating the memory with vmalloc().

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