metag: perf: use hard_processor_id() to get thread

Use hard_processor_id() to get the current thread number rather than
get_cpu() and the hardware thread mapping. There was no matching
put_cpu(), and in any case this should be slightly more efficient.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>