nfs41: Implement NFSv4.1 callback service process.

nfs41_callback_up() initializes the necessary queues and creates the new
nfs41_callback_svc thread.  This thread executes the callback service which
waits for requests to arrive on the svc_serv->sv_cb_list.

NFS41_BC_MIN_CALLBACKS is set to 1 because we expect callbacks to not
cause substantial latency.

The actual processing of the callback will be implemented as a separate patch.

There is only one NFSv4.1 callback service.  The first caller of
nfs4_callback_up() creates the service, subsequent callers increment a
reference count on the service.  The service is destroyed when the last
caller invokes nfs_callback_down().

The transport needs to hold a reference to the callback service in order
to invoke it during callback processing.  Currently this reference is only
obtained when the service is first created.  This is incorrect, since
subsequent registrations for other transports will leave the xprt->serv
pointer uninitialized, leading to an oops when a callback arrives on
the "unreferenced" transport.

This patch fixes the problem by ensuring that a reference to the service
is saved in xprt->serv, either because the service is created by this
invocation to nfs4_callback_up() or by a prior invocation.

Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: Add a reference to svc_serv during callback service bring up]
Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[Type check arguments of nfs_callback_up]
Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: save svc_serv in nfs_callback_info]
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[Removal of ugly #ifdefs]
[nfs41: Update to removal of ugly #ifdefs]
Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>