xprtrdma: Make xprt_rdma_slot_table_entries static

Fix the following sparse warning:

net/sunrpc/xprtrdma/transport.c:71:14: warning: symbol 'xprt_rdma_slot_table_entries'
was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>