afs: Make it possible to find the volumes that are using a server

[ Upstream commit ca0e79a4 ]

Make it possible to find the afs_volume structs that are using an
afs_server struct to aid in breaking volume callbacks.

The way this is done is that each afs_volume already has an array of
afs_server_entry records that point to the servers where that volume might
be found.  An afs_volume backpointer and a list node is added to each entry
and each entry is then added to an RCU-traversable list on the afs_server
to which it points.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Stable-dep-of: add117e4 ("afs: Fix the server_list to unuse a displaced server rather than putting it")
Signed-off-by: Sasha Levin <sashal@kernel.org>