afs: Don't cross .backup mountpoint from backup volume

Don't cross a mountpoint that explicitly specifies a backup volume
(target is <vol>.backup) when starting from a backup volume.

It it not uncommon to mount a volume's backup directly in the volume
itself.  This can cause tools that are not paying attention to get
into a loop mounting the volume onto itself as they attempt to
traverse the tree, leading to a variety of problems.

This doesn't prevent the general case of loops in a sequence of
mountpoints, but addresses a common special case in the same way
as other afs clients.

Reported-by: Jan Henrik Sylvester <jan.henrik.sylvester@uni-hamburg.de>
Link: http://lists.infradead.org/pipermail/linux-afs/2024-May/008454.html
Reported-by: Markus Suvanto <markus.suvanto@gmail.com>
Link: http://lists.infradead.org/pipermail/linux-afs/2024-February/008074.html
Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/768760.1716567475@warthog.procyon.org.uk
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>