build failure due to upstream linux change

[cpackham]

Hi,

We just bumped our linux kernel from 6.6.75 to 6.6.76 and netmap started failing to build.

awplus-arx200,linux: In file included from /usr/src/netmap-974b68c0/LINUX/../sys/dev/netmap/netmap_vale.c:58:
awplus-arx200,linux: /usr/src/netmap-974b68c0/LINUX/../sys/dev/netmap/netmap_vale.c: In function 'netmap_vi_create':
awplus-arx200,linux: /usr/src/netmap-974b68c0/LINUX/bsd_glue.h:345:26: error: assignment to 'struct ax25_dev *' from incompatible pointer type 'struct netmap_adapter *' [-Werror=incompatible-pointer-types]
awplus-arx200,linux:   345 |         (_ifp)->ax25_ptr = _na;                                         \
awplus-arx200,linux:       |                          ^
awplus-arx200,linux: /usr/src/netmap-974b68c/LINUX/../sys/dev/netmap/netmap_kern.h:1723:9: note: in expansion of macro 'if_setnetmapadapter'
awplus-arx200,linux:  1723 |         if_setnetmapadapter(ifp, na);                                   \
awplus-arx200,linux:       |         ^~~~~~~~~~~~~~~~~~~
awplus-arx200,linux: /usr/src/netmap-974b68c0/LINUX/../sys/dev/netmap/netmap_vale.c:1476:9: note: in expansion of macro 'NM_ATTACH_NA'
awplus-arx200,linux:  1476 |         NM_ATTACH_NA(ifp, &vpna->up);
awplus-arx200,linux:       |         ^~~~~~~~~~~~

Looks like this is due to the upstream change https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-6.6.y&id=7705d8a7f2c26c80973c81093db07c6022b2b30e which changed net_device::ax25_ptr from a void * to struct ax25_dev __rcu *