kmemleak: Save the stack trace for early allocations

Before slab is initialised, kmemleak save the allocations in an early
log buffer. They are later recorded as normal memory allocations. This
patch adds the stack trace saving to the early log buffer, otherwise the
information shown for such objects only refers to the kmemleak_init()
function.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>