arm64: entry: refine comment of stack overflow check

Stack overflow checking can be done by testing sp & (1 << THREAD_SHIFT)
only for the stacks are aligned to (2 << THREAD_SHIFT) with size of
(1 << THREAD_SIZE), and this is the case when CONFIG_VMAP_STACK is set.

Fix the code comment to avoid confusion.

Cc: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Heyi Guo <guoheyi@huawei.com>
[catalin.marinas@arm.com: Updated comment following Mark's suggestion]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -76,7 +76,8 @@ alternative_else_nop_endif
 #ifdef CONFIG_VMAP_STACK
 	/*
 	 * Test whether the SP has overflowed, without corrupting a GPR.
-	 * Task and IRQ stacks are aligned to (1 << THREAD_SHIFT).
+	 * Task and IRQ stacks are aligned so that SP & (1 << THREAD_SHIFT)
+	 * should always be zero.
 	 */
 	add	sp, sp, x0			// sp' = sp + x0
 	sub	x0, sp, x0			// x0' = sp' - x0 = (sp + x0) - x0 = sp