NFSv41: Fix nfs4_proc_create_session

We should not assume that nfs41_init_clientid() will always want to
initialise the session. If it is being called due to a server reboot, then
we just want to reset the session after re-establishing the clientid.

Fix this by getting rid of the 'reset' parameter in
nfs4_proc_create_session(), and instead relying on whether or not the
session slot table pointer is non-NULL.

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