afs: Fix afs_server_list to be cleaned up with RCU

afs: Fix afs_server_list to be cleaned up with RCU

[ Upstream commit e6bace7313d61e31f2b16fa3d774fd8cb3cb869e ]

afs_server_list is accessed with the rcu_read_lock() held from
volume->servers, so it needs to be cleaned up correctly.

Fix this by using kfree_rcu() instead of kfree().

Fixes: 8a070a964877 ("afs: Detect cell aliases 1 - Cells with root volumes")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Sasha Levin <sashal@kernel.org>