datapath: Improve robustness of this_cpu_ptr definition in compat layer

Current autoconfig detection logic for HAVE_PER_CPU_PTR is not robust.
Depends on linux kernel version, the definition can be in either
linux/percpu.h or asm/percpu.h

Turns out it is simpler and safer to handle missing percpu.h
definitions in linux/percpu.h rather than asm/percpu.h. With this
change, there is no need for the autoconfig detection logic above.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>