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>

net/sunrpc/xprtrdma/transport.c

@@ -68,7 +68,7 @@
  * tunables
  */
 
-unsigned int xprt_rdma_slot_table_entries = RPCRDMA_DEF_SLOT_TABLE;
+static unsigned int xprt_rdma_slot_table_entries = RPCRDMA_DEF_SLOT_TABLE;
 unsigned int xprt_rdma_max_inline_read = RPCRDMA_DEF_INLINE;
 unsigned int xprt_rdma_max_inline_write = RPCRDMA_DEF_INLINE;
 unsigned int xprt_rdma_memreg_strategy		= RPCRDMA_FRWR;