svc: Change sk_inuse to a kref

svc: Change sk_inuse to a kref

Change the atomic_t reference count to a kref and move it to the
transport indepenent svc_xprt structure. Change the reference count
wrapper names to be generic.

Signed-off-by: Tom Tucker <[email protected]>
Acked-by: Neil Brown <[email protected]>
Reviewed-by: Chuck Lever <[email protected]>
Reviewed-by: Greg Banks <[email protected]>
Signed-off-by: J. Bruce Fields <[email protected]>