nfsd: convert nfs4_cb_conn struct to hold address in sockaddr_storage

...rather than as a separate address and port fields. This will be
necessary for implementing callbacks over IPv6. Also, convert
gen_callback to use the standard rpcuaddr2sockaddr routine rather than
its own private one.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>