netmap: silence -Wdefault-const-init-field-unsafe warning
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
Reviewed by: vmaffione, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52568