The DEFINE_PER_CPU of current_pgd was on CRIS defined using volatile,
which is not needed. Remove volatile.

Tested on an ARTPEC-3 (CRISv32) board.

tj: extern DEFINE_PER_CPU() replaced with DECLARE_PER_CPU()

[ Impact: code cleanup ]
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>