powerpc: Do not assign thread.tidr if already assigned

If set_thread_tidr() is called twice for same task_struct then it will
allocate a new tidr value to it leaving the previous value still
dangling in the vas_thread_ida table.

To fix this the patch changes set_thread_tidr() to check if a tidr
value is already assigned to the task_struct and if yes then returns
zero.

Fixes: ec233ede

("powerpc: Add support for setting SPRN_TIDR")
Signed-off-by: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
[mpe: Modify to return 0 in the success case, not the TID value]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>