netmap: silence -Wdefault-const-init-field-unsafe warning

Summary:
The netmap_ring struct starts with various const members and rencent
clang warns about leaving them uninitialized. Having them const in the
first place is highly suspicious since they are updated with various
macros but using hand-coded __DECONST(). But fixing that is a more
invasive change that I am unable to test.

```
.../freebsd/sys/dev/netmap/netmap_kloop.c:320:21: error: default initialization of an object of type 'struct netmap_ring' with const member leaves the object uninitialized [-Werror,-Wdefault-const-init-field-unsafe]
  320 |         struct netmap_ring shadow_ring; /* shadow copy of the netmap_ring */
      |                            ^
.../freebsd/sys/net/netmap.h:290:16: note: member 'buf_ofs' declared 'const' here
  290 |         const int64_t   buf_ofs;
      |                         ^
```

Test Plan: Compiles

Reviewers: vmaffione, jhb

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D52568

Author: arichardson

sys/dev/netmap/netmap_kloop.c

@@ -161,7 +161,8 @@ netmap_sync_kloop_tx_ring(const struct sync_kloop_ring_args *a)
 	struct netmap_kring *kring = a->kring;
 	struct nm_csb_atok *csb_atok = a->csb_atok;
 	struct nm_csb_ktoa *csb_ktoa = a->csb_ktoa;
-	struct netmap_ring shadow_ring; /* shadow copy of the netmap_ring */
+	/* shadow copy of the netmap_ring */
+	struct netmap_ring shadow_ring = {0};
 #ifdef SYNC_KLOOP_POLL
 	bool more_txspace = false;
 #endif /* SYNC_KLOOP_POLL */
@@ -317,7 +318,8 @@ netmap_sync_kloop_rx_ring(const struct sync_kloop_ring_args *a)
 	struct netmap_kring *kring = a->kring;
 	struct nm_csb_atok *csb_atok = a->csb_atok;
 	struct nm_csb_ktoa *csb_ktoa = a->csb_ktoa;
-	struct netmap_ring shadow_ring; /* shadow copy of the netmap_ring */
+	/* shadow copy of the netmap_ring */
+	struct netmap_ring shadow_ring = {0};
 	int dry_cycles = 0;
 #ifdef SYNC_KLOOP_POLL
 	bool some_recvd = false;