tipc: rename the module name diag to tipc_diag

tipc: rename the module name diag to tipc_diag

It is not appropriate for TIPC to use "diag" as its diag module name
while the other protocols are using "$(protoname)_diag" like tcp_diag,
udp_diag and sctp_diag etc.

So this patch is to rename diag.ko to tipc_diag.ko in tipc's Makefile.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Link: https://lore.kernel.org/r/d909edeef072da1810bd5869fdbbfe84411efdb2.1706904669.git.lucien.xin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>