UPSTREAM: fs: drop peer group ids under namespace lock

commit cb2239c1 upstream.

When cleaning up peer group ids in the failure path we need to make sure
to hold on to the namespace lock. Otherwise another thread might just
turn the mount from a shared into a non-shared mount concurrently.

Link: https://lore.kernel.org/lkml/00000000000088694505f8132d77@google.com


Fixes: 2a186721 ("fs: add mount_setattr()")
Reported-by:  <syzbot+8ac3859139c685c4f597@syzkaller.appspotmail.com>
Cc: stable@vger.kernel.org # 5.12+
Message-Id: <20230330-vfs-mount_setattr-propagation-fix-v1-1-37548d91533b@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Bug: 275196365
Change-Id: I0075d0057b61ed4a32a87a394c966ff0235cccdc
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>