ieee802154: fix kernel-infoleak in dgram_recvmsg()

In the Linux kernel, the following vulnerability has been resolved: ieee802154: fix kernel-infoleak in dgram_recvmsg() KMSAN reported a kernel-infoleak in move_addr_to_user(): BUG: KMSAN: kernel-infoleak in instrument_copy_to_user include/linux/instrumented.h:131 [inline] BUG: KMSAN: kernel-infoleak in _inline_copy_to_user include/linux/uaccess.h:205 [inline] BUG: KMSAN: kernel-infoleak in _copy_to_user+0xcc/0x120 lib/usercopy.c:26 instrument_copy_to_user include/linux/instrumented.h:131 [inline] _inline_copy_to_user include/linux/uaccess.h:205 [inline] _copy_to_user+0xcc/0x120 lib/usercopy.c:26 copy_to_user include/linux/uaccess.h:236 [inline] move_addr_to_user+0x2e7/0x440 net/socket.c:302 ____sys_recvmsg+0x232/0x610 net/socket.c:2925 ... Uninit was stored to memory at: ieee802154_addr_to_sa include/net/ieee802154_netdev.h:369 [inline] dgram_recvmsg+0xa09/0xbe0 net/ieee802154/socket.c:739 The issue occurs because the `pan_id` field of `struct ieee802154_addr` is left uninitialized when the address mode is `IEEE802154_ADDR_NONE`. The execution flow is as follows: 1. `__ieee802154_rx_handle_packet()` declares a local `struct ieee802154_hdr hdr` on the stack. 2. `ieee802154_hdr_pull()` calls `ieee802154_hdr_get_addr()` to parse the source and destination addresses into this structure. 3. If the address mode is `IEEE802154_ADDR_NONE`, `ieee802154_hdr_get_addr()` previously only set the `mode` field, leaving the `pan_id` field containing uninitialized stack memory. 4. This uninitialized `pan_id` is later copied into a `struct sockaddr_ieee802154` in `dgram_recvmsg()` via `ieee802154_addr_to_sa()`. 5. Finally, `move_addr_to_user()` copies the socket address structure to user space, leaking the uninitialized bytes. Fix this by using `memset` to zero out the address structure in `ieee802154_hdr_get_addr()` when the mode is `IEEE802154_ADDR_NONE`.