SUNRPC: Change the default limit to the number of TCP slots

Since the scheme of limiting the number of TCP slots to whatever will
fit in the current TCP window seems to be working well (Andy reports
getting within 20% of the 'iperf' send performance on a 10GigE link)
we should just let that be the default mode of operation.

Users may still set their own limits using the tcp_max_slot_table_entries
parameter if they need to.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>