netfilter: nftables: comment indirect serialization of commit_mutex with rtnl_mutex

Add an explicit comment in the code to describe the indirect
serialization of the holders of the commit_mutex with the rtnl_mutex.
Commit 90d2723c ("netfilter: nf_tables: do not hold reference on
netdevice from preparation phase") already describes this, but a comment
in this case is better for reference.

Reported-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>