svcrdma: Pre-allocate svc_rdma_recv_ctxt objects

The original reason for allocating svc_rdma_recv_ctxt objects during
Receive completion was to ensure the objects were allocated on the
NUMA node closest to the underlying IB device.

Since commit c5d68d25

 ("svcrdma: Clean up allocation of
svc_rdma_recv_ctxt"), however, the device's favored node is
explicitly passed to the memory allocator.

To enable switching Receive completion to soft IRQ context, move
memory allocation out of completion handling, since it can be
costly, and it can sleep.

A limited number of objects is now allocated at "accept" time.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>