Add a reference to sunrpc in svc_addsock

The svc_addsock function adds transport instances without taking a
reference on the sunrpc.ko module, however, the generic transport
destruction code drops a reference when a transport instance
is destroyed.

Add a try_module_get call to the svc_addsock function for transport
instances added by this function.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Tested-by: Jeff Moyer <jmoyer@redhat.com>