nfs41: Backchannel: Refactor nfs4_init_slot_table()

Generalize nfs4_init_slot_table() so it can be used to initialize the
backchannel slot table in addition to the forechannel slot table.

Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>