x86/mm: Check return value from memblock_phys_alloc_range()

In the Linux kernel, the following vulnerability has been resolved:

x86/mm: Check return value from memblock_phys_alloc_range()

At least with CONFIG_PHYSICAL_START=0x100000, if there is < 4 MiB of

contiguous free memory available at this point, the kernel will crash

and burn because memblock_phys_alloc_range() returns 0 on failure,

which leads memblock_phys_free() to throw the first 4 MiB of physical

memory to the wolves.

At a minimum it should fail gracefully with a meaningful diagnostic,

but in fact everything seems to work fine without the weird reserve

allocation.