gve: guard XSK operations on the existence of queues

In the Linux kernel, the following vulnerability has been resolved: gve: guard XSK operations on the existence of queues This patch predicates the enabling and disabling of XSK pools on the existence of queues. As it stands, if the interface is down, disabling or enabling XSK pools would result in a crash, as the RX queue pointer would be NULL. XSK pool registration will occur as part of the next interface up. Similarly, xsk_wakeup needs be guarded against queues disappearing while the function is executing, so a check against the GVE_PRIV_FLAGS_NAPI_ENABLED flag is added to synchronize with the disabling of the bit and the synchronize_net() in gve_turndown.