SUNRPC: Make the credential cache hashtable size configurable

This patch allows the user to configure the credential cache hashtable size
using a new module parameter: auth_hashtable_size
When set, this parameter will be rounded up to the nearest power of two,
with a maximum allowed value of 1024 elements.

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