LoongArch: Change initrd allocation to top-down

When loading a secondary kernel with KASLR enabled, the randomized relocation
of the kernel image may overlap the initrd memory region, causing data corruption
and boot failure. This occurs because KASLR dynamically shifts the kernel's load
address, which could accidentally target the initrd's pre-allocated space.

Modify the initrd allocation strategy from bottom-up to top-down within the
reserved memory region. By placing the initrd at higher addresses first, we
minimize the risk of the relocated kernel overwriting it.

Reported-by: Chenghao Duan <duanchenghao@kylinos.cn>
Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Simon Horman <horms@kernel.org>