SUNRPC: rework cache upcall logic

For most of SUNRPC caches (except NFS DNS cache) cache_detail->cache_upcall is
redundant since all that it's implementations are doing is calling
sunrpc_cache_pipe_upcall() with proper function address argument.
Cache request function address is now stored on cache_detail structure and
thus all the code can be simplified.
Now, for those cache details, which doesn't have cache_upcall callback (the
only one, which still has is nfs_dns_resolve_template)
sunrpc_cache_pipe_upcall will be called instead.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>