page-alloc/x86: don't restrict DMA heap to node 0

page-alloc/x86: don't restrict DMA heap to node 0

When node zero has no memory, the DMA bit width will end up getting set
to 9, which is obviously not helpful to hold back a reasonable amount
of low enough memory for Dom0 to use for DMA purposes. Find the lowest
node with memory below 4Gb instead.

Introduce arch_get_dma_bitsize() to keep this arch-specific logic out
of common code.

Also adjust the original calculation: I think the subtraction of 1
should have been part of the flsl() argument rather than getting
applied to its result. And while previously the division by 4 was valid
to be done on the flsl() result, this now also needs to be converted,
as is should only be applied to the spanned pages value.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>