wifi: mac80211: Replace strncpy() with strscpy_pad() in drv_switch_vif_chanctx tracepoint

wifi: mac80211: Replace strncpy() with strscpy_pad() in drv_switch_vif_chanctx tracepoint

Replace the deprecated[1] strncpy() with strscpy_pad() for copying the
interface name into a tracepoint entry.

The source "sdata->name" is a NUL-terminated char[IFNAMSIZ] buffer
populated via NUL-guaranteeing paths: strscpy() in ieee80211_if_add(),
snprintf() in ieee80211_add_virtual_monitor(), or memcpy() from
ndev->name in ieee80211_if_add() and netdev_notify()
(net/mac80211/iface.c). In the memcpy() cases, the source ndev->name
is itself always NUL-terminated (populated via snprintf() or strscpy()
in __dev_alloc_name() and dev_prep_valid_name() in net/core/dev.c).

The destination "local_vifs[i].vif.vif_name" is a char[IFNAMSIZ] field
in struct trace_vif_entry, stored in a __dynamic_array within the trace
ring buffer. Since ring buffer entries are not zeroed on allocation,
strscpy_pad() is used to zero-fill trailing bytes and prevent exposing
stale ring buffer contents to userspace readers of tracefs.

No behavioral change: since interface names are always at most 15
characters plus a NUL terminator, strscpy_pad() with size IFNAMSIZ (16)
produces identical output to the original strncpy().

Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20260323172015.work.146-kees@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>