svcrdma: Clean up RPCRDMA_DEF_GFP

xprt_rdma_bc_allocate() is now the only user of RPCRDMA_DEF_GFP.
Replace that macro with the raw flags.

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