svcrdma: Fix incorrect return value/type in svc_rdma_post_recvs

This crept in during the development process and wasn't caught
before I posted the "final" version.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 0b2613c5883f ('svcrdma: Allocate recv_ctxt's on CPU ... ')
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>