SIMD: Use alloc_pages_node to force alignment

fxsave and xsave require the target address to be 16-/64-byte aligned.

kmalloc(_node) does not (yet) offer such fine-grained control over
alignment[0,1], even though it does "the right thing" most of the time
for power-of-2 sizes. unfortunately, alignment is completely off when
using certain debugging or hardening features/configs, such as KASAN,
slub_debug=Z or the not-yet-upstream SLAB_CANARY.

Use alloc_pages_node() instead which allows us to allocate page-aligned
memory. Since fpregs_state is padded to a full page anyway, and this
code is only relevant for x86 which has 4k pages, this approach should
not allocate any unnecessary memory but still guarantee the needed
alignment.

0: https://lwn.net/Articles/787740/
1: https://lore.kernel.org/linux-block/[email protected]/

Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Fabian Grünbichler <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #9608
Closes #9674

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