wifi: mac80211: Fix overread in PREP frame processing

wifi: mac80211: Fix overread in PREP frame processing

When the AF flag is enabled, hwmp_prep_frame_process() overreads orig_addr
by 2 bytes. Since this occurs within the socket buffer, it does not read
across memory boundaries and therefore poses no security risk; however, we
will fix it as a precaution.

In this fix, a new function mesh_path_parse_reply_frame() is established to
separate the implementation of frame format validation and the check for
unsupported features. This is intended to facilitate future work when
implementing the currently unsupported parts.

Assisted-by: Claude:Sonnet 4.6
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Link: https://patch.msgid.link/20260529230952.124754-5-masashi.honma@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>