mm/mempool.c: update the kmemleak stack trace for mempool allocations

mm/mempool.c: update the kmemleak stack trace for mempool allocations

When mempool_alloc() returns an existing pool object, kmemleak_alloc() is
no longer called and the stack trace corresponds to the original object
allocation.  This patch updates the kmemleak allocation stack trace for
such objects to make it more useful for debugging.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>