high_memory used to be initialized in mem_init, way after setup_bootmem.
But a call to dma_contiguous_reserve in this function gives rise to the
below warning because high_memory is equal to 0 and is used at the very
beginning at cma_declare_contiguous_nid.
It went unnoticed since the move of the kasan region redefined
KERN_VIRT_SIZE so that it does not encompass -1 anymore.
Fix this by initializing high_memory in setup_bootmem.