arm64: Honor nosmp kernel command line option

Passing "nosmp" should boot the kernel with a single processor, without
provision to enable secondary CPUs even if they are present. "nosmp" is
implemented by setting maxcpus=0. At the moment we still mark the secondary
CPUs present even with nosmp, which allows the userspace to bring them
up. This patch corrects the smp_prepare_cpus() to honor the maxcpus == 0.

Commit 44dbcc93ab67145 ("arm64: Fix behavior of maxcpus=N") fixed the
behavior for maxcpus >= 1, but broke maxcpus = 0.

Fixes: 44dbcc93ab67 ("arm64: Fix behavior of maxcpus=N")
Cc: <stable@vger.kernel.org> # 4.7+
Cc: Will Deacon <will.deacon@arm.com>
Cc: James Morse <james.morse@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
[catalin.marinas@arm.com: updated code comment]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/smp.c

@@ -692,6 +692,13 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 
 	smp_store_cpu_info(smp_processor_id());
 
+	/*
+	 * If UP is mandated by "nosmp" (which implies "maxcpus=0"), don't set
+	 * secondary CPUs present.
+	 */
+	if (max_cpus == 0)
+		return;
+
 	/*
 	 * Initialise the present map (which describes the set of CPUs
 	 * actually populated at the present time) and release the