percpu: kill compile warning in pcpu_populate_chunk()

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: Tejun Heo <tj@kernel.org>
Reported-by: Ingo Molnar <mingo@elte.hu>