i40e: use a mutex instead of spinlock in PTP user entry points

We need a locking mechanism to protect the hardware SYSTIME register
which is split over 2 values, and has internal hardware latching. We
can't allow multiple accesses at the same time. However....

The spinlock_t is overkill here, especially use of spin_lock_irqsave,
since every PTP access will halt hardirqs. Notice that the only places
which need the SYSTIME value are user context and are capable of sleeping.
Thus, it is safe to use a mutex here instead of the spinlock.

Change-ID: I971761a89b58c6aad953590162e85a327fbba232
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e.h

@@ -430,7 +430,7 @@ struct i40e_pf {
 	struct sk_buff *ptp_tx_skb;
 	struct hwtstamp_config tstamp_config;
 	unsigned long last_rx_ptp_check;
-	spinlock_t tmreg_lock; /* Used to protect the device time registers. */
+	struct mutex tmreg_lock; /* Used to protect the SYSTIME registers. */
 	u64 ptp_base_adj;
 	u32 tx_hwtstamp_timeouts;
 	u32 rx_hwtstamp_cleared;