sunrpc: do not pin sunrpc module in the memory

Basically, try_module_get here are pretty useless. Any other module using
this API will pin sunrpc in memory due using exported symbols.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>