netlink: do not free nlk->groups while lockless readers can use it

In the Linux kernel, the following vulnerability has been resolved:

netlink: do not free nlk->groups while lockless readers can use it

netlink_realloc_groups() uses krealloc() under netlink_table_grab().
Whenever NLGRPSZ(groups) lands in a different kmalloc bucket, the old
bitmap is freed immediately.

Two readers of nlk->groups / nlk->ngroups do not hold the netlink
table lock:

1) sk_diag_dump_groups(). Hashed (bound) sockets are dumped from the
rhashtable walk in __netlink_diag_dump(), which only holds RCU.
Only the mc_list part of the dump takes nl_table_lock.

2) netlink_native_seq_show() (/proc/net/netlink), whose walk has been
lockless since commit 21e4902aea80 ("netlink: Lockless lookup with
RCU grace period in socket release").

Both can read a freed buffer, and sk_diag_dump_groups() can also read
past the end of the old (smaller) buffer if it happens to load the old
@groups pointer together with the new @ngroups value, copying the
result into a NETLINK_DIAG_GROUPS attribute.

This is the same class of bug that commit f773608026ee ("netlink:
access nlk groups safely in netlink bind and getname") fixed for bind()
and getname(); these two readers were missed. Simply grabbing the table
lock in sk_diag_dump_groups() is not an option, because it is also
called with nl_table_lock already held from the mc_list section of the
dump.

Make the lockless readers safe instead:

- Allocate a new bitmap and free the old one after an RCU grace period,
instead of relying on the implicit kfree() done by krealloc().

- Publish @groups before @ngroups, both with release semantics, and have
the lockless readers load @ngroups first. A reader can then never pair
the new (bigger) size with the old (smaller) buffer, and a reader
picking up the new pointer while still seeing the old size is
guaranteed to see the initialized bitmap.

netlink_realloc_groups() is called from process context (bind() and
setsockopt()), so kfree_rcu_mightsleep() can be used, once the table
has been released.