afs: Fix afs_get_link() to take validate_lock around afs_read_single()

Fix afs_get_link() to take the validate_lock around afs_read_single() to
prevent races between multiple ->get_link() calls.

Fixes: eae9e78951bb ("afs: Use netfslib for symlinks, allowing them to be cached")
Closes: https://sashiko.dev/#/patchset/20260326104544.509518-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org