percpu: replace pcpu_realloc() with pcpu_mem_alloc() and pcpu_mem_free()

Impact: code reorganization for later changes

With static map handling moved to pcpu_split_block(), pcpu_realloc()
only clutters the code and it's also unsuitable for scheduled locking
changes.  Implement and use pcpu_mem_alloc/free() instead.

Signed-off-by: Tejun Heo <tj@kernel.org>