malloc(9): drop the __result_use_check attribute for the kernel allocator.

The __result_use_check attribute was brought to the kernel malloc in
r281203 for consistency with the userland malloc.

For the case of the M_WAITOK flag, the kernel malloc(), realloc(), and
reallocf() cannot return NULL so in that case the __result_use_check
attribute makes no sense.

We don't have any way of conditionalizing such attributes so just drop it.

MFC after: 3 days