percpu: Generic support for this_cpu_cmpxchg_double()

percpu: Generic support for this_cpu_cmpxchg_double()

Introduce this_cpu_cmpxchg_double().  this_cpu_cmpxchg_double() allows
the comparison between two consecutive words and replaces them if
there is a match.

	bool this_cpu_cmpxchg_double(pcp1, pcp2,
		old_word1, old_word2, new_word1, new_word2)

this_cpu_cmpxchg_double does not return the old value (difficult since
there are two words) but a boolean indicating if the operation was
successful.

The first percpu variable must be double word aligned!

-tj: Updated to return bool instead of int, converted size check to
     BUILD_BUG_ON() instead of VM_BUG_ON() and other cosmetic changes.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>