SUNRPC: Fix an unnecessary implicit type cast in rpcrdma_count_chunks()

Nit: rl_nchunks is an unsigned integer, so pass it into
rpcrdma_count_chunks() via an unsigned integer argument.  This eliminates
a harmless mixed sign comparison in rpcrdma_count_chunks()

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Thomas Talpey <Thomas.Talpey@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>