svcrdma: Remove unused variables in xprt_rdma_bc_allocate()

Clean up.

/linux-2.6/net/sunrpc/xprtrdma/svc_rdma_backchannel.c: In function
 ‘xprt_rdma_bc_allocate’:
linux-2.6/net/sunrpc/xprtrdma/svc_rdma_backchannel.c:169:23: warning:
 variable ‘rdma’ set but not used [-Wunused-but-set-variable]
  struct svcxprt_rdma *rdma;
                       ^

Fixes: 5d252f90 ("svcrdma: Add class for RDMA backwards ...")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>