KVM: arm64: Drop useless __GFP_HIGHMEM from kvm struct allocation

KVM: arm64: Drop useless __GFP_HIGHMEM from kvm struct allocation

A recent change on the receiving end of vmalloc() started warning about
unsupported GFP flags passed by the caller. Nathan reports that this
warning fires in kvm_arch_alloc_vm(), owing to the fact that KVM is
passing a meaningless __GFP_HIGHMEM.

Do as the warning says and fix the code.

Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/kvmarm/20251118224448.GA998046@ax162/
Acked-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://msgid.link/20251119093822.2513142-2-oupton@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>