Revert "net: Add a mutex around devnet_rename_seq"

This reverts commit 65bc42b6.

In mainline, commit 11d6011c ("net: core: device_rename: Use rwsem
instead of a seqcount") appeared in v5.8-rc1, as an rt-friendly patch.

If we look in the upstream linux-rt-devel repository, we see that in
deploying it in v5.6-rt, as fast forward commit 1984aa963358, we see it
1st appeared as part of a 24 commit series in v5.6.17-rt10:

   commit 11bf6eb48f4db2117b9eb4875245a68c0340ca92 (tag: v5.6.17-rt10-patches)
   Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
   Date:   Tue Jun 16 14:30:28 2020 +0200

       [ANNOUNCE] v5.6.17-rt10

       Dear RT folks!

       I'm pleased to announce the v5.6.17-rt10 patch set.

       Changes since v5.6.17-rt9:

         - Add the seqcount series by Ahmed S. Darwish.
           The series annotates the lock protecting the struct seqcount against
           multiple writer. In RT the lock annotation can be utilized to block
           on in case the writer is active. This is the same technique that is
           used now but we can avoid replacing the seqcount with a seqlock and
           have code that is closer to mainline.

In preparation, we see in the fast-forward, commit 3115f1749777 ("Revert
seqcount related patches") which is a bulk revert of about 12 earlier
preempt-rt commits, including a revert of commit 969f7336673c ("net: Add
a mutex around devnet_rename_seq")

The v5.2.49 kernel also contains a backport of 11d6011c, and so we'll
run into a merge conflict as the two -rt devnet_rename solutions overlap.

So prior to merging v5.2.49 to v5.2-rt, we revert the older -rt solution
in order to make way for the newer one from the v5.2.49 kernel, just as
we see in the above v5.6-rt version.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>