x86/hyper-v: Add "polling" bit to hv_synic_sint

That bit is documented in TLFS 5.0c as follows:

  Setting the polling bit will have the effect of unmasking an
  interrupt source, except that an actual interrupt is not generated.
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20191222233404.1629-1-wei.liu@kernel.org

arch/x86/include/asm/hyperv-tlfs.h

@@ -809,7 +809,8 @@ union hv_synic_sint {
 		u64 reserved1:8;
 		u64 masked:1;
 		u64 auto_eoi:1;
-		u64 reserved2:46;
+		u64 polling:1;
+		u64 reserved2:45;
 	} __packed;
 };