virtio-net: free xsk_buffs on error in virtnet_xsk_pool_enable()

virtio-net: free xsk_buffs on error in virtnet_xsk_pool_enable()

The selftests added to our CI by Bui Quang Minh recently reveals
that there is a mem leak on the error path of virtnet_xsk_pool_enable():

unreferenced object 0xffff88800a68a000 (size 2048):
  comm "xdp_helper", pid 318, jiffies 4294692778
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 0):
    __kvmalloc_node_noprof+0x402/0x570
    virtnet_xsk_pool_enable+0x293/0x6a0 (drivers/net/virtio_net.c:5882)
    xp_assign_dev+0x369/0x670 (net/xdp/xsk_buff_pool.c:226)
    xsk_bind+0x6a5/0x1ae0
    __sys_bind+0x15e/0x230
    __x64_sys_bind+0x72/0xb0
    do_syscall_64+0xc1/0x1d0
    entry_SYSCALL_64_after_hwframe+0x77/0x7f

Acked-by: Jason Wang <jasowang@redhat.com>
Fixes: e9f3962441c0 ("virtio_net: xsk: rx: support fill with xsk buffer")
Link: https://patch.msgid.link/20250430163836.3029761-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>