MIPS: Loongson64: Add WARN_ON on kexec related kmalloc failed

Add WARN_ON on kexec related kmalloc failed, avoid to pass NULL pointer
to following memcpy and loongson_kexec_prepare.

Fixes: 6ce48897 ("MIPS: Loongson64: Add kexec/kdump support")
Signed-off-by: Liao Chang <liaochang1@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>