memblock tests: update zeroed memory check for memblock_alloc_* tests

Update the assert in memblock_alloc_try_nid() and memblock_alloc_from()
tests that checks whether the memory is cleared so that it checks the
entire chunk of allocated memory instead of just the first byte.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Shaoqin Huang <shaoqin.huang@intel.com>
Signed-off-by: Rebecca Mckeever <remckee0@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/24b3271751756100142e65b75284d43b4d30c9b7.1661578349.git.remckee0@gmail.com