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.