x86: Use kmemdup() in copy_thread(), rather than duplicating its implementation

The semantic patch that makes this change is available
in scripts/coccinelle/api/memdup.cocci.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Link: http://lkml.kernel.org/r/1321569820.1624.275.camel@localhost.localdomain
Signed-off-by: Ingo Molnar <mingo@elte.hu>