In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Use memset_after() so memset() doesn't get confused about writing
beyond the destination member that is intended to be the starting point
of zeroing through the end of the struct.

Additionally fix the common helper, ieee80211_tx_info_clear_status(),
which was not clearing ack_signal, but the open-coded versions
did. Johannes Berg points out this bug was introduced by commit
e3e1a0bc ("mac80211: reduce IEEE80211_TX_MAX_RATES") but was harmless.

Also drops the associated unneeded BUILD_BUG_ON()s, and adds a note to
carl9170 about usage.
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Christian Lamparter <chunkeey@gmail.com> [both CARL9170+P54USB on real HW]
Link: https://lore.kernel.org/r/20211118203839.1289276-1-keescook@chromium.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>