selftests/bpf: Fix maybe-uninitialized warning in xdpxceiver test

xsk_ring_prod__reserve() doesn't necessarily set idx in some conditions, so
from static analysis point of view compiler is right about the problems like:

In file included from xdpxceiver.c:92:
xdpxceiver.c: In function ‘xsk_populate_fill_ring’:
/data/users/andriin/linux/tools/testing/selftests/bpf/tools/include/bpf/xsk.h:119:20: warning: ‘idx’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  return &addrs[idx & fill->mask];
                ~~~~^~~~~~~~~~~~
xdpxceiver.c:300:6: note: ‘idx’ was declared here
  u32 idx;
      ^~~
xdpxceiver.c: In function ‘tx_only’:
xdpxceiver.c:596:30: warning: ‘idx’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   struct xdp_desc *tx_desc = xsk_ring_prod__tx_desc(&xsk->tx, idx + i);
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix two warnings reported by compiler by pre-initializing variable.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210313210920.1959628-4-andrii@kernel.org

tools/testing/selftests/bpf/xdpxceiver.c

@@ -297,7 +297,7 @@ static void xsk_configure_umem(struct ifobject *data, void *buffer, u64 size)
 static void xsk_populate_fill_ring(struct xsk_umem_info *umem)
 {
 	int ret, i;
-	u32 idx;
+	u32 idx = 0;
 
 	ret = xsk_ring_prod__reserve(&umem->fq, XSK_RING_PROD__DEFAULT_NUM_DESCS, &idx);
 	if (ret != XSK_RING_PROD__DEFAULT_NUM_DESCS)
@@ -584,7 +584,7 @@ static void rx_pkt(struct xsk_socket_info *xsk, struct pollfd *fds)
 
 static void tx_only(struct xsk_socket_info *xsk, u32 *frameptr, int batch_size)
 {
-	u32 idx;
+	u32 idx = 0;
 	unsigned int i;
 	bool tx_invalid_test = stat_test_type == STAT_TEST_TX_INVALID;
 	u32 len = tx_invalid_test ? XSK_UMEM__DEFAULT_FRAME_SIZE + 1 : PKT_SIZE;