bpf: Check flow_dissector ctx accesses are aligned

flow_dissector_is_valid_access doesn't check that the context access is
aligned. As a consequence, an unaligned access within one of the exposed
field is considered valid and later rejected by
flow_dissector_convert_ctx_access when we try to convert it.

The later rejection is problematic because it's reported as a verifier
bug with a kernel warning and doesn't point to the right instruction in
verifier logs.

Fixes: d58e468b ("flow_dissector: implements flow dissector BPF hook")
Reported-by:  <syzbot+ccac90e482b2a81d74aa@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=ccac90e482b2a81d74aa


Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/cc1b036be484c99be45eddf48bd78cc6f72839b1.1754039605.git.paul.chaignon@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>