Impact: remove compile warning

Mark local variable map_end in pcpu_populate_chunk() with
uninitialized_var().  The variable is always used in tandem with
map_start and guaranteed to be initialized before use but gcc doesn't
understand that.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NIngo Molnar <mingo@elte.hu>