x86_64: make reserve_bootmem_generic() use new reserve_bootmem()

"mm: make reserve_bootmem can crossed the nodes" provides new
reserve_bootmem(), let reserve_bootmem_generic() use that.

reserve_bootmem_generic() is used to reserve initramdisk, so this way
we can make sure even when bootloader or kexec load ranges cross the
node memory boundaries, reserve_bootmem still works.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>