net: genetlink: always allocate separate attrs for dumpit ops

Individual dumpit ops (start, dumpit, done) are locked by genl_lock
if !family->parallel_ops. However, multiple
genl_family_rcv_msg_dumpit() calls may in in flight in parallel.
Each has a separate struct genl_dumpit_info allocated
but they share the same family->attrbuf. Fix this by allocating separate
memory for attrs for dumpit ops, for non-parallel_ops (for parallel_ops
it is done already).

Reported-by:  <syzbot+495688b736534bb6c6ad@syzkaller.appspotmail.com>
Reported-by:  <syzbot+ff59dc711f2cff879a05@syzkaller.appspotmail.com>
Reported-by:  <syzbot+dbe02e13bcce52bcf182@syzkaller.appspotmail.com>
Reported-by:  <syzbot+9cb7edb2906ea1e83006@syzkaller.appspotmail.com>
Fixes: bf813b0a

 ("net: genetlink: parse attrs and store in contect info struct during dumpit")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>