xsk: avoid double checking against rx queue being full

Currently non-zc xsk rx path for multi-buffer case checks twice if xsk
rx queue has enough space for producing descriptors:
1.
	if (xskq_prod_nb_free(xs->rx, num_desc) < num_desc) {
		xs->rx_queue_full++;
		return -ENOBUFS;
	}
2.
	__xsk_rcv_zc(xs, xskb, copied - meta_len, rem ? XDP_PKT_CONTD : 0);
	-> err = xskq_prod_reserve_desc(xs->rx, addr, len, flags);
	  -> if (xskq_prod_is_full(q))

Second part is redundant as in 1. we already peeked onto rx queue and
checked that there is enough space to produce given amount of
descriptors.

Provide helper functions that will skip it and therefore optimize code.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://lore.kernel.org/r/20260218150000.301176-1-maciej.fijalkowski@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>