If percpu pointer is const, __verify_pcpu_ptr() triggers warnings like
the following.

 drivers/net/loopback.c: In function 'loopback_get_stats':
 drivers/net/loopback.c:109: warning: initialization discards qualifiers from pointer target type

Fix it by adding const to the verification target pointer used in
__verify_pcpu_ptr().
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>