virtio_net: Fix napi_skb_cache_put warning

In the Linux kernel, the following vulnerability has been resolved:

virtio_net: Fix napi_skb_cache_put warning

After the commit bdacf3e34945 ("net: Use nested-BH locking for

napi_alloc_cache.") was merged, the following warning began to appear:

WARNING: CPU: 5 PID: 1 at net/core/skbuff.c:1451 napi_skb_cache_put+0x82/0x4b0

__warn+0x12f/0x340

napi_skb_cache_put+0x82/0x4b0

napi_skb_cache_put+0x82/0x4b0

report_bug+0x165/0x370

handle_bug+0x3d/0x80

exc_invalid_op+0x1a/0x50

asm_exc_invalid_op+0x1a/0x20

__free_old_xmit+0x1c8/0x510

napi_skb_cache_put+0x82/0x4b0

__free_old_xmit+0x1c8/0x510

__free_old_xmit+0x1c8/0x510

__pfx___free_old_xmit+0x10/0x10

The issue arises because virtio is assuming it's running in NAPI context

even when it's not, such as in the netpoll case.

To resolve this, modify virtnet_poll_tx() to only set NAPI when budget

is available. Same for virtnet_poll_cleantx(), which always assumed that

it was in a NAPI context.