ice: fix UAF/NULL deref when VSI rebuild and XDP attach race

ice_xdp_setup_prog() unconditionally hot-swaps xdp_prog when
ICE_VSI_REBUILD_PENDING is set. In the attach path, this can publish a
new rx_ring->xdp_prog before rx_ring->xdp_ring becomes valid while the
rebuild is pending. As a result, ice_clean_rx_irq() may dereference
rx_ring->xdp_ring too early.

With high-volume RX packets, running these commands in parallel
triggered a KASAN splat [1].
 # ethtool --reset $DEV irq dma filter offload
 # ip link set dev $DEV xdp {obj $OBJ sec xdp,off}

Fix this by rejecting XDP attach while rebuild is pending.
Keep XDP detach allowed in this window. Detach clears rx_ring->xdp_prog,
so the RX path will not attempt to access rx_ring->xdp_ring.

[1]
BUG: KASAN: slab-use-after-free in ice_napi_poll+0x3921/0x41a0
Read of size 2 at addr ffff88812475b880 by task ksoftirqd/1/23
[...]
Call Trace:
 <TASK>
 ice_napi_poll+0x3921/0x41a0
 __napi_poll+0x98/0x520
 net_rx_action+0x8f2/0xfa0
 handle_softirqs+0x1cb/0x7f0
[...]
 </TASK>

Allocated by task 7246:
 ice_prepare_xdp_rings+0x3de/0x12d0
 ice_xdp+0x61c/0xef0
 dev_xdp_install+0x3c4/0x840
 dev_xdp_attach+0x50a/0x10a0
 dev_change_xdp_fd+0x175/0x210
[...]

Freed by task 7251:
 __rcu_free_sheaf_prepare+0x5f/0x230
 rcu_free_sheaf+0x1a/0xf0
 rcu_core+0x567/0x1d80
 handle_softirqs+0x1cb/0x7f0

Fixes: 2504b8405768 ("ice: protect XDP configuration with a mutex")
Signed-off-by: Kohei Enju <kohei@enjuk.jp>