i40e: free the skb after clearing the bitlock

In commit bbc4e7d2 ("i40e: fix race condition with PTP_TX_IN_PROGRESS
bits") we modified the code which handles Tx timestamps so that we would
clear the progress bit as soon as possible.

A later commit 0bc0706b ("i40e: check for Tx timestamp timeouts during
watchdog") introduced similar code for detecting and handling cleanup of
a blocked Tx timestamp. This code did not use the same pattern for cleaning
up the skb.

Update this code to wait to free the skb until after the bit lock is
free, by first setting the ptp_tx_skb to NULL and clearing the lock.

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>